Fix: Suggest a new username

If the user picks a take username then nodebb will suggest that username + "suffix"

Author: josecmu27

public/language/en-US/error.json

@@ -32,7 +32,7 @@
     "invalid-path": "Invalid path",
     "folder-exists": "Folder exists",
     "invalid-pagination-value": "Invalid pagination value, must be at least %1 and at most %2",
-    "username-taken": "Username taken",
+    "username-taken": "Username taken. Maybe try %1",
     "email-taken": "Email address is already taken.",
     "email-nochange": "The email entered is the same as the email already on file.",
     "email-invited": "Email was already invited",

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-taken, "${username}suffix"]]`);
 				}
 
 				callback();