Check to see if function exists before calling it.

includes/derivatives.inc

@@ -60,11 +60,16 @@ function islandora_do_derivatives(AbstractObject $object, array $options) {
       require_once $hook['file'];
     }
     foreach ($hook['function'] as $function) {
+      if (function_exists($function)) {
         $logging = call_user_func($function, $object, $options['force']);
         if (!empty($logging)) {
           $results[] = $logging;
         }
       }
+      else {
+        watchdog('islandora', 'Unable to call derivative function @function as it was not found!', array('@function' => $function), WATCHDOG_ERROR);
+      }
+    }
   }
   return $results;
 }