Error messages: return JSON instead of raw messages
When answering for http://metab.ucoin.io/wot/certifiers-of/keythatdoesnotexist, we should:
- get an HTTP 404 error instead of HTTP 400
- get a typical JSON error response instead of raw message
When answering for http://metab.ucoin.io/wot/certifiers-of/keythatdoesnotexist, we should: