spray ExceptionHandler not handling exceptions

[flibbertigibbet]

Although there is an exception handler defined in the implicit scope for the spray routes, it doesn't appear to actually catch errors. The handler translates its exception into JSON to send back, but since it's not catching exceptions, celery is getting an unexpected response, resulting in the UI displaying the error message "No JSON object could be decoded: line 1 column 0 (char 0)".