Utilize `override_messages` in JWT authentication middleware

[olenotron]

Is your feature request related to a problem? Please describe.
override_messages lets users customize error code and messages globally. While it works with AuthKey and oauth2, it is not supported in JWT auth.

Describe the solution you'd like
Since, for example, I can already set custom errors for AuthKey like this:

{
    "override_messages": {
      "auth.auth_field_missing": {
        "code": 401,
        "message": "Authorization field missing"
      }
    }
}

I want to be able to set custom errors for JWT auth like this:

{
    "override_messages": {
      "jwt.auth_field_missing": {
        "code": 401,
        "message": "Authorization field missing"
      }
    }
}

Additional context
I'll be glad to create a PR if you agree with the enhancement. I won't rehaul all the errors, only the ones that look easily replaceable with standard ones.

[probelabs]

Hello @olenotron, thank you for this feature request and for offering to contribute a pull request!

Understanding

I understand that you'd like to extend the override_messages functionality to the JWT authentication middleware (mw_jwt.go). This would allow for consistent and customizable error responses across different authentication methods, bringing the JWT middleware in line with how AuthKey and OAuth2 authentications handle errors.

Analysis

This is a great suggestion for improving the consistency and user-friendliness of Tyk Gateway's error handling. The ability to customize error messages is important, and it makes sense to have this capability available for JWT-based APIs.

Based on your description and the existing implementation in other middleware, your proposed approach seems correct. The changes would likely involve:

1. Identifying the error-producing code paths in gateway/mw_jwt.go.
2. Defining new error keys specific to JWT, such as jwt.auth_field_missing, jwt.token_expired, etc.
3. Replacing the hardcoded error responses with calls to a centralized error handling function that respects the override_messages configuration.

Next Steps

We would welcome a PR for this enhancement. As you work on it, please consider the following:

• Error Keys: It would be beneficial to review all error responses in gateway/mw_jwt.go and map them to a comprehensive set of overridable error keys.
• Consistency: Please ensure the implementation follows the pattern used in gateway/mw_auth_key.go and gateway/mw_oauth2_key_exists.go for consistency.
• Testing: Be sure to add unit tests to verify that the override_messages for JWT errors are working as expected.

We look forward to your contribution!

References

• I was unable to access the codebase to provide specific line references, but the files you mentioned are the correct ones to investigate: gateway/mw_jwt.go, gateway/mw_auth_key.go, and gateway/mw_oauth2_key_exists.go.

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