Add message for UUID validation rule

See: c96995fb61 (diff-9c04f727a922ad4f105b9ad764ec221d)

extensions/lang-english/locale/validation.yml

@@ -90,6 +90,7 @@ validation:
   unique: "The :attribute has already been taken."
   uploaded: "The :attribute failed to upload."
   url: "The :attribute format is invalid."
+  uuid: "The :attribute must be a valid UUID."
 
   attributes:
     username: username