MDL-53274 core: in AJAX throw exception if page context was not set

lib/pagelib.php

@@ -443,10 +443,15 @@ class moodle_page {
      * @return context the main context to which this page belongs.
      */
     protected function magic_get_context() {
+        global $CFG;
         if (is_null($this->_context)) {
             if (CLI_SCRIPT or NO_MOODLE_COOKIES) {
                 // Cli scripts work in system context, do not annoy devs with debug info.
                 // Very few scripts do not use cookies, we can safely use system as default context there.
+            } else if (AJAX_SCRIPT && $CFG->debugdeveloper) {
+                // Throw exception inside AJAX script in developer mode, otherwise the debugging message may be missed.
+                throw new coding_exception('$PAGE->context was not set. You may have forgotten '
+                    .'to call require_login() or $PAGE->set_context()');
             } else {
                 debugging('Coding problem: $PAGE->context was not set. You may have forgotten '
                     .'to call require_login() or $PAGE->set_context(). The page may not display '