Better HTTP error handling on application submission

[petschekr]

For example, when a resume that is too large is uploaded, a 413 Request Entity Too Large error is returned by the file upload middleware. The current code tries to parse the HTML error page as JSON resulting in an opaque Unexpected token < in JSON at position 0 error.

Better error handling would involve user-friendly error messages for common HTTP status codes and a catch-all handler for this that displays the malformed JSON for debugging (e.g. in a "Show details" link)