fix: fix the data structure of `RepositoryResponse` when it represents an error

[Colk-tech]

The error message mentions 'self.reason' but the validation is actually checking for 'self.message'. The message should say "'self.message' is required when 'self.is_success' is set to 'ERROR'" to accurately reflect what's being validated.

                "'self.message' is required when 'self.is_success' is set to 'ERROR'"

Originally posted by @Copilot in #7 (comment)