[#3] Raise a 400 for invalid JSON

- Added new method to listener, `decodeJson()`; instead of just decoding
  the JSON, it also checks for errors, and, if any are found, raises a
  `DomainException` with a relevant error message and a 400 code.
[#3] CS fixes

- Trailing whitespace
Merge branch 'hotfix/3'

Close #3