Added suggestion for alternative username if chosen username taken at registration in register.js

public/src/client/register.js

@@ -131,7 +131,10 @@
                 if (results.every(obj => obj.status === 'rejected')) {
                     showSuccess(username_notify, successIcon);
                 } else {
-                    showError(username_notify, '[[error:username-taken]]');
+                    // Username taken, suggest alternative 
+                    const suggested = username + 'suffix'; 
+                    const suggestionMsg = '[[error:username-taken]]<br>Suggested: <span class="username-suggestion">' + suggested + '</span>'; 
+                    showError(username_notify, suggestionMsg);
                 }
 
                 callback();