Error codes

The HTTP transport maps the fediversity.error.* codes to HTTP status codes:

Code HTTP status
fediversity.error.not_found 404
fediversity.error.invalid_configuration 400
fediversity.error.queue_full 409
fediversity.error.conflict 409
fediversity.error.unauthorized 401
fediversity.error.authz_denied 403
fediversity.error.service_unavailable 503
fediversity.error.build_timeout 504
fediversity.error.runtime_error 500