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.
Built from https://develop.svn.wordpress.org/trunk@51126


git-svn-id: http://core.svn.wordpress.org/trunk@50735 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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
+			)
 		);
 	}
 

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.8-alpha-51125';
+$wp_version = '5.8-alpha-51126';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.