[ticket/11279] Clearer AJAX errors

Display clearer AJAX errors rather than generic error

PHPBB3-11279
[ticket/11279] Log error in console

PHPBB-11279
Merge remote-tracking branch 'remotes/cyberalien/ticket/11279' into develop

# By Vjacheslav Trushkin
# Via Vjacheslav Trushkin
* remotes/cyberalien/ticket/11279:
  [ticket/11279] Log error in console
  [ticket/11279] Clearer AJAX errors