Prevent artefact commits while Mahara is in the middle of crashing.

This is the #1 cause of Fatal Errors in Mahara - when a second exception (usually an SQLException) is thrown while we're in the middle of handling one already.

Now the first exception gets to follow through to its conclusion, which results in far better error messages.

htdocs/artefact/lib.php

@@ -313,7 +313,7 @@ abstract class ArtefactType {
             return;
         }
       
-        if (!empty($this->dirty)) {
+        if (!empty($this->dirty) && !defined('MAHARA_CRASHING')) {
             $this->commit();
         }
     }

htdocs/lib/errors.php

@@ -423,6 +423,13 @@ class MaharaException extends Exception {
 
     protected $log = true;
 
+    public function __construct($message, $code=0) {
+        parent::__construct($message, $code);
+        if (!defined('MAHARA_CRASHING')) {
+            define('MAHARA_CRASHING', true);
+        }
+    }
+
     public function get_string() {
         $args = func_get_args();
         if (function_exists('get_string')) {