expand error handling

[cwulfman]

To the maximum extent possible, the API should perform error checking on requests, and should always return a meaningful status, either with the resulting data or an indication that the request was invalid or that the data was not available.

[cwulfman]

• The API now checks for malformed input (bad bmtnids) and returns 400 if the bmtnid isn't valid.
• The API returns 204 if the result of a query is empty.
• The null path (i.e., .../springs) now re-directs to the documention of the API on GitHub.