idl: Disallow multiple error definitions

[acheroncrypto]

Problem

Having multiple error definitions is not supported, but you don't get any errors when you do have them. Instead, the compiler chooses one of the definitions and discards the rest.

Summary of changes

Make having multiple error definitions a hard error during IDL generation.

Progress

• Add multiple-errors tests that checks for the desired behavior (error when there are multiple error definitions i.e. enums with #[error_code]) (CI):

  1) multiple-errors
       Returns 'multiple errors' error on builds:
     Error: No error on build
  

• Disallow multiple error definitions

Fixes #3532

[vercel]

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