[Agency] Avoid custom http status code

[HenriJ]

The current version of the specification (https://github.com/CityOfLosAngeles/mobility-data-specification/blob/e2faabf5a7e59f4ec4422a1e0c8c4e66361547cc/agency/README.md) is not very clear about what the structure of the response body should be in case of an error.
It looks like the "message" enum are http status code ? If it is the case, using custom http status code should be avoided (especially using 3xx codes for client errors)

2 examples of good responses body in case of error:

• How Oauth2 does it: https://tools.ietf.org/html/rfc6749 { "error": "my_error_code", "error_description": "human readable message" }
• How a Kafka subsystem does it (add 2 digits to http status codes): https://github.com/confluentinc/kafka-rest/blob/master/docs/api.rst#errors {"error_code": "40401", "message": "human readable message"}

[oderby]

Dupe of #105

[hunterowens]

closing as dupe. Move to #105