Clarify username taken error message during registration

Author: belugaaaaaaaa

public/language/en-US/error.json

@@ -264,5 +264,7 @@
     "activitypub.pubKey-not-found": "Unable to resolve public key, so payload verification cannot take place.",
     "activitypub.origin-mismatch": "The received object's origin does not match the sender's origin",
     "activitypub.actor-mismatch": "The received activity is being carried out by an actor that is different from expected.",
-    "activitypub.not-implemented": "The request was denied because it or an aspect of it is not implemented by the recipient server"
+    "activitypub.not-implemented": "The request was denied because it or an aspect of it is not implemented by the recipient server",
+    "username-or-group-taken": "Username unavailable (conflicts with an existing user or group).",
+
 }

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();