Allow custom error message for json schema validator

Currently, the json_schema validator doesn't support a custom message. It returns a generic 'Validation failed: <attr> must be a valid json schema'. Update the validator to allow a custom message.

So, ideally I could use the validator in a model:

validates :job_token_policies, json_schema: { filename: 'ci_job_token_policies', message: 'custom error message' }