Better error when calling `get_taped_globals` outside a TapedTask

[mhauru]

julia> using Libtask

julia> Libtask.get_taped_globals(Any)
ERROR: KeyError: key :task_variable not found
Stacktrace:
 [1] getindex
   @ ./iddict.jl:98 [inlined]
 [2] task_local_storage
   @ ./task.jl:293 [inlined]
 [3] get_taped_globals(::Type{Any})
   @ Libtask ~/projects/Libtask.jl/src/copyable_task.jl:21
 [4] top-level scope
   @ REPL[2]:1

[penelopeysm]

With #215:

julia> using Libtask; Libtask.get_taped_globals(String)
ERROR: Libtask.NotInTapedTaskError: The method `get_taped_globals(::Type)` can only be called from inside a TapedTask.
Stacktrace:
 [1] get_taped_globals(::Type{String})
   @ Libtask ~/ppl/Libtask.jl/src/copyable_task.jl:34
 [2] top-level scope
   @ REPL[1]:1

It's also a specific error struct instead of some generic KeyError, which makes it possible for a caller to catch it and handle it cleverly.