[elixir] Support fine-grained errors like in other bindings

[fresh-borzoni]

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• I searched in the issues and found nothing similar.

Description

The Elixir bindings currently return {:error, String.t()} for all errors, losing the structured error information available in the Rust core. The Rust/Python and C++ bindings already expose error codes - the Elixir binding should have parity.

Willingness to contribute

• I'm willing to submit a PR!