Log fatal errors for AJAX responses. CRM-15560

If you have a fatal error in an AJAX call, then the error doesn't get
logged to ConfigAndLog/Civicrm.*.log. This is because the logging stuff
happens after the call to CRM_Core_Page_AJAX::returnJsonResponse in
CRM_Core_Error::fatal and returnJsonRespons exits php.
Merge pull request #4511 from giant-rabbit/ajax-fatal-log-fix

Log fatal errors for AJAX responses. CRM-15560