FIX Don't overwrite JSON errors with templated error page

[GuySartorelli]

The new error handling logic was overwriting JSON error messages, which are most likely responses for AJAX requests. A more complete fix would be to find out why they weren't being detected as AJAX requests and resolve that - but that could lead to even more regressions, and would be time consuming besides.

Parent issue

• Regressions caused by new error handling in LeftAndMain #1332

[emteknetnz]

This seems appropriate as we use the "hardcoded" string 'application/json' elsewhere in LeftAndMain.js to denote XHR requests

HTTPRequest::isAjax() looks like a sub-optimal method as it requires setting querystring or header values client side, instead of a smarter automatic detection, so likely it's giving the wrong result in many instances

HTTPRequest.php

    public function isAjax()
    {
        return (
            $this->requestVar('ajax') ||
            $this->getHeader('x-requested-with') === "XMLHttpRequest"
        );
    }