JSON response for too short prefix

[pohlm01]

The Routinator API currently returns a plaintext response Bad Request together with the HTTP status 400 Bad Request in case the requested prefix is too short. See for example https://routinator.do.nlnetlabs.nl/api/v1/validity/AS15169/104.154.0.0/14.

It would be great to have a more explanatory error response, preferably in JSON format, such that a helpful error message can be displayed to the user in the UI. The HTTP status code should stay an error code, e.g., 400 Bad Request, in my opinion.

Additionally, the API currently does not send any CORS-headers in case of this failure. Therefore, we end up with an CORS-error in the browser logs. This is probably not directly connected to Routinator, but more to the deployment. Either way, having the headers set correctly, is required to read out the new JSON error response.