Fixes #8837 - Return correctly formatted response on ajax_error

[tbrisker]

No description provided.

[dLobatog]

Merged as de3b8fe, thanks @tbrisker

I tested this by adding a process_ajax_error StandardError.new('sadasd'), 'fetch overview information' line at the beginning of hosts_controller : overview and visiting a Host page, then putting a breakpoint in the if(status == 'error') condition part of application.js. That's in the data-ajax-url fetching .each loop. It returns the right error status and message as JSON. I removed the logger.debug line to that prints the backtrace when testing.