Login and Registration: Improve the unknown username error message

The new message is more clear and makes the next step more clear to the user. 

Props dd32, SergeyBiryukov, audrasjb, desrosj, dlh, marybaum, chanthaboune, jeffpaul.
Fixes #52915.

git-svn-id: https://develop.svn.wordpress.org/trunk@51126 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/user.php

@@ -149,7 +149,11 @@ function wp_authenticate_username_password( $user, $username, $password ) {
 	if ( ! $user ) {
 		return new WP_Error(
 			'invalid_username',
-			__( '<strong>Error</strong>: Unknown username. Check again or try your email address.' )
+			sprintf(
+				/* translators: %s: User name. */
+				__( '<strong>Error</strong>: The username <strong>%s</strong> is not registered on this site. If you are unsure of your username, try your email address instead.' ),
+				$username
+			)
 		);
 	}