Improve error message when trying to link `:manual` output pad to `:push` input pad

[kidq330]

While the docs discourage using :push input pads, in the rare case they are needed, it would help to have a better error message to understand the location of the problem in the pipeline quicker - example:

Mix.install( [ {:membrane_core, "~> 1.0"}, {:membrane_generator_plugin, ">= 0.0.0"}] )

defmodule MyFilter do
  use Membrane.Filter

  def_input_pad :input,
    accepted_format: _any,
    flow_control: :push

  def_output_pad :output,
    accepted_format: _any,
    flow_control: :manual, demand_unit: :buffers

  @impl true
  def handle_init(_ctx, _opts) do
    {[], %{}}
  end

  @impl true
  def handle_buffer(_pad, buffer, _ctx, state) do
    {[forward: buffer], state}
  end

end


defmodule MyPipeline do
  use Membrane.Pipeline

  @impl true
  def handle_init(_ctx, _opts) do
    {[spec:
        [
          # NOTE: `Membrane.SilenceGenerator`'s output pad has `flow_control: :manual`
          child(:silence, %Membrane.SilenceGenerator{
            duration: :infinity,
            stream_format: %Membrane.RawAudio{
              channels: 1,
              sample_rate: 16_000,
              sample_format: :s16le
            }
          })
          |> child(:filter, MyFilter)
          |> child(:fake_sink, Membrane.Fake.Sink)
        ]
    ], %{}}
  end
end

{:ok, _supervisor, _pipeline} = Membrane.Pipeline.start_link(MyPipeline, [])
Process.sleep(:infinity)

Error message:

11:37:45.026 [error] <0.167.0>/:filter Error occured in Membrane Element:
** (Membrane.LinkError) Cannot connect :manual output :output to push input :input
    (membrane_core 1.2.6) lib/membrane/core/child/pad_controller.ex:33: Membrane.Core.Child.PadController.validate_pads_flow_control_compability!/4
    (membrane_core 1.2.6) lib/membrane/core/element/pad_controller.ex:182: Membrane.Core.Element.PadController.do_handle_link/6
    (membrane_core 1.2.6) lib/membrane/core/element.ex:192: Membrane.Core.Element.do_handle_call/3
    (membrane_core 1.2.6) lib/membrane/core/element.ex:179: Membrane.Core.Element.handle_call/3
    (stdlib 7.2.1) gen_server.erl:2470: :gen_server.try_handle_call/4
    (stdlib 7.2.1) gen_server.erl:2499: :gen_server.handle_msg/3
    (stdlib 7.2.1) proc_lib.erl:333: :proc_lib.init_p_do_apply/3



11:37:45.026 [error] GenServer #PID<0.175.0> terminating
** (Membrane.LinkError) Cannot connect :manual output :output to push input :input
    (membrane_core 1.2.6) lib/membrane/core/child/pad_controller.ex:33: Membrane.Core.Child.PadController.validate_pads_flow_control_compability!/4
    (membrane_core 1.2.6) lib/membrane/core/element/pad_controller.ex:182: Membrane.Core.Element.PadController.do_handle_link/6
    (membrane_core 1.2.6) lib/membrane/core/element.ex:192: Membrane.Core.Element.do_handle_call/3
    (membrane_core 1.2.6) lib/membrane/core/element.ex:179: Membrane.Core.Element.handle_call/3
    (stdlib 7.2.1) gen_server.erl:2470: :gen_server.try_handle_call/4
    (stdlib 7.2.1) gen_server.erl:2499: :gen_server.handle_msg/3
    (stdlib 7.2.1) proc_lib.erl:333: :proc_lib.init_p_do_apply/3
Last message (from #PID<0.172.0>): {Membrane.Core.Message, :handle_link, [:input, %Membrane.Core.Parent.Link.Endpoint{child: :filter, pad_spec: :input, pad_ref: :input, pid: #PID<0.175.0>, pad_props: %{options: [], target_queue_size: nil, min_demand_factor: nil, auto_demand_size: nil, toilet_capacity: nil, throttling_factor: nil}, pad_info: %{name: :input, options: [], flow_control: :push, direction: :input, accepted_formats_str: ["_any"], availability: :always}, child_spec_ref: #Reference<0.3220357644.3508011010.51619>}, %Membrane.Core.Parent.Link.Endpoint{child: :silence, pad_spec: :output, pad_ref: :output, pid: #PID<0.172.0>, pad_props: %{options: []}, pad_info: %{name: :output, options: [], flow_control: :manual, demand_unit: nil, direction: :output, accepted_formats_str: ["RawAudio"], availability: :always}, child_spec_ref: #Reference<0.3220357644.3508011010.51619>}, %{stream_format_validation_params: [], link_metadata: %{observability_data: %{path: ["<0.167.0>/", ":silence"], observer_dbg_process: nil}}, output_effective_flow_control: :pull, output_pad_info: %{name: :output, options: [], flow_control: :manual, demand_unit: nil, direction: :output, accepted_formats_str: ["RawAudio"], availability: :always}}], []}
State: %Membrane.Core.Element.State{module: MyFilter, name: :filter, parent_pid: #PID<0.167.0>, playback: :stopped, type: :filter, internal_state: %{}, pads_info: %{input: %{name: :input, options: [], flow_control: :push, direction: :input, accepted_formats_str: ["_any"], availability: :always}, output: %{name: :output, options: [], flow_control: :manual, demand_unit: :buffers, direction: :output, accepted_formats_str: ["_any"], availability: :always}}, synchronization: %{timers: %{}, clock: nil, stream_sync: :membrane_no_sync, parent_clock: #PID<0.170.0>, latency: 0}, delayed_demands: MapSet.new([]), effective_flow_control: :push, initialized?: true, terminating?: false, setup_incomplete_returned?: false, delay_demands?: false, popping_auto_flow_queue?: false, stalker: %Membrane.Core.Stalker{pid: #PID<0.168.0>, ets: #Reference<0.3220357644.3508142082.51566>}, resource_guard: #PID<0.176.0>, subprocess_supervisor: #PID<0.174.0>, handle_demand_loop_counter: 0, pads_to_snapshot: MapSet.new([]), playback_queue: [], diamond_detection_state: %Membrane.Core.Element.DiamondDetectionController.DiamondDatectionState{ref_to_path: %{}, trigger_refs: MapSet.new([]), postponed?: false}, pads_data: %{}, satisfied_auto_output_pads: MapSet.new([]), awaiting_auto_input_pads: MapSet.new([]), auto_input_pads: [], resume_delayed_demands_loop_in_mailbox?: false}
Client #PID<0.172.0> is alive

    (stdlib 7.2.1) gen.erl:262: :gen.do_call/4
    (elixir 1.19.5) lib/gen_server.ex:1139: GenServer.call/3
    (membrane_core 1.2.6) lib/membrane/core/message.ex:39: Membrane.Core.Message.call/5
    (membrane_core 1.2.6) lib/membrane/core/element/pad_controller.ex:85: Membrane.Core.Element.PadController.do_handle_link/6
    (membrane_core 1.2.6) lib/membrane/core/element.ex:192: Membrane.Core.Element.do_handle_call/3
    (membrane_core 1.2.6) lib/membrane/core/element.ex:179: Membrane.Core.Element.handle_call/3
    (stdlib 7.2.1) gen_server.erl:2470: :gen_server.try_handle_call/4
    (stdlib 7.2.1) gen_server.erl:2499: :gen_server.handle_msg/3

The error message notes :filter as one of the offending elements, but it would help if it also listed :silence as the upstream offender.