Response headers and helpful messages

The JSON objects returned by `jsonify` did not specify a CORS response
header, so AJAX requests to the API were being shut down for security
reasons. This update adds the CORS-friendly header to the response
object returned from `jsonify` and then returns that update object.

Also there were some 500 errors and empty responses generated if you hit
an endpoint without the proper parameters. This update also adds some
helpful messages to API requests that don't have the necessary params.