replace 2 error messages with keys

public/src/app.js

@@ -320,25 +320,11 @@ var socket,
 
 	app.openChat = function (username, touid) {
 		if (username === app.username) {
-			app.alert({
-				type: 'warning',
-				title: 'Invalid Chat',
-				message: "You can't chat with yourself!",
-				timeout: 5000
-			});
-
-			return;
+			return app.alertError('[[error:cant-chat-with-yourself]]');
 		}
 
 		if (!app.uid) {
-			app.alert({
-				type: 'danger',
-				title: 'Not Logged In',
-				message: 'Please log in to chat with <strong>' + username + '</strong>',
-				timeout: 5000
-			});
-
-			return;
+			return app.alertError('[[error:not-logged-in]]');
 		}
 
 		require(['chat'], function (chat) {