Return 500 code in JSON rather than actual 500 error

[allanjamesvestal]

When something goes wrong with a geocoding request, we should return a JSON object with the 5XX code (and a description of the error), rather than returning an actual HTTP 5XX response.

This will let other applications handle errors within AJAX success() functions, and should make those integrations less brittle in general.

I suppose we could also add an option to return error objects rather than actual statuses, in case developers prefer to handle 5XX codes in a more browser-native way.