Updated mass assignment exception wording for clarity; nominating the…

… key alone is a little vague

src/Illuminate/Database/Eloquent/Model.php

@@ -229,7 +229,7 @@ public function fill(array $attributes)
             if ($this->isFillable($key)) {
                 $this->setAttribute($key, $value);
             } elseif ($totallyGuarded) {
-                throw new MassAssignmentException($key);
+                throw new MassAssignmentException('Model is guarded. Add key ['.$key.'] to $fillable to allow mass assignment.');
             }
         }