wiggle: delete GuestErrorConversion, improve some error reporting

[pchickey]

With the latest renovation to witx, wiggle no longer generates code that calls the GuestErrorConversion trait to transform a GuestError into the function's error type. Instead, wiggle will trap execution if a GuestError is thrown in generated code.

This PR removes generation of the GuestErrorConversion trait. Users can delete their impls of it - they are dead code.

I also fixed up the Display impl of the GuestError::InFunc variant to be more useful, and deleted GuestError::InDataField because it was also dead code.

[github-actions]

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