How to Hide WordPress Login Error Messages
The message that says “Invalid username” could potentially help a hacker in his search for the right username and password.
You can easily hide these messages by adding the following snippet to your theme’s functions.php file. This tip comes to you courtesy of Shahzad Saeed who posted it over at isitwp.com:
add_filter('login_errors', create_function('$a', "return null;"));
This will help your site be a little more secure in the face of a direct hacking attempt.