Provide a public enum for `sys::GDExtensionCallErrorType`

[TitanNano]

The ScriptInstance makes use of the internal sys::GDExtensionCallErrorType to communicate the call error of a script method call. This should be exposed as a proper rust enum instead of a type alias for u32 with constants as variants.

This will greatly improve the user experience of implementing ScriptInstance::call as the type and possible variants will actually be available to the implementor.