Clarify username taken error message during registration

Author: TonyHeeee

public/language/en-US/error.json

@@ -19,6 +19,7 @@
     "invalid-fullname": "Invalid Fullname",
     "invalid-location": "Invalid Location",
     "invalid-birthday": "Invalid Birthday",
+    "username-or-group-taken": "Username is already taken by a user or a group.",
     "invalid-title": "Invalid title",
     "invalid-user-data": "Invalid User Data",
     "invalid-password": "Invalid Password",

public/src/client/register.js

@@ -135,7 +135,7 @@ define('forum/register', [
 				if (results.every(obj => obj.status === 'rejected')) {
 					showSuccess(usernameInput, username_notify, successIcon);
 				} else {
-					showError(usernameInput, username_notify, '[[error:username-taken]]');
+					showError(usernameInput, username_notify, '[[error:username-or-group-taken]]');
 				}
 
 				callback();