API errors are not valid JSON

[romuald]

It seems most of the errors are not returned as JSON but at text/html

% curl -D/dev/stderr http://baobab.local/api/events'?some-filter'

HTTP/1.1 500 INTERNAL SERVER ERROR
Content-Type: text/html; charset=utf-8
Date: Tue, 04 Oct 2016 16:45:48 GMT
Server: nginx
Vary: Cookie
Content-Length: 31
Connection: keep-alive

Sorry but something went wrong

The error should probably be wrapped inside simple JSON structure