Catch fatal errors after cycling page action

This is a complex issues where a "model not found" exception is getting thrown during the `pageAction` cycle, then getting suppressed by the fatal error handler:

try {
    // RelationController throws error here
}
catch (Exception $ex) {
    $this->controller->handleError($ex);
}

Fixes #4784

modules/backend/behaviors/RelationController.php

@@ -290,6 +290,10 @@ class RelationController extends ControllerBehavior
         }
 
         $this->controller->pageAction();
+        if ($fatalError = $this->controller->getFatalError()) {
+            throw new ApplicationException($fatalError);
+        }
+
         $this->validateField();
         $this->prepareVars();
         $this->initialized = true;