Turn off PHP errors display for all ajax calls

Whenever there is an ajax call, any PHP errors displayed will disrupt the output and make the interpretation of the ajax output fail on the client side.

See CRM-15893 and many other issues.

While we can fix each of these issues individually it might be better, as Coleman suggested, to turn off php errors display on all ajax calls. In doing so, we should filter to only do this on ajax calls, and not interfere with php error reporting directed to a log file by changing error reporting level or other parameters.