Expose add_error_handler() to GDExtensions

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Describe the project you are working on

Some functions in godot return void, with the only indication that an error happened with a call to all registered error handlers.

One example is GDExtension::_register_extension_class_method, which returns void, but silently fails if a method with the same name was already registered.

Internally in godot, there's an ErrorDetector.

It would be extremely useful to be able to use the same mechanism in an gdextension, rust in my case.

Describe the problem or limitation you are having in your project

Some errors are silently ignored when testing a GDExtension, for example in gdext-rust, I accidentally registered a method twice, and I only noticed this because of a memory leak.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

Expose the two functions add_error_handler and remove_error_handler to GDExtensions

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

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If this enhancement will not be used often, can it be worked around with a few lines of script?

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Is there a reason why this should be core and not an add-on in the asset library?

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