MDL-73905 core: fix coding_exception missing namespace

This was leading to exception:

Class 'core\message\coding_exception' not found

Surely we weren't facing this before MDL-67853, but it seems that
now we are hitting it sometimes (being investigated @ MDL-73747).

So, just ensure the proper \coding_exception is used.

lib/classes/message/manager.php

@@ -228,7 +228,7 @@ public static function send_message_to_conversation(message $eventdata, \stdClas
                         // exist in the message_provider table (thus there is no default settings for them).
                         $preferrormsg = "Could not load preference $defaultlockedpreference.
                      Make sure the component and name you supplied to message_send() are valid.";
-                        throw new coding_exception($preferrormsg);
+                        throw new \coding_exception($preferrormsg);
                     }
 
                     $enabledpreference = 'message_provider_'.$preferencebase . '_enabled';