Login and Registration: Improve "email already exists" registration error message.

Adds the `wp_login_url()` login link to the Error message to be more helpful to users when their user email already is registered.

Improves the error message to more clearly communicate next step.

Follow-up to [16009], [22124], [31963].

Props andynick, costdev, dansoschin, sabernhardt, webcommsat.
Fixes #53631.
Built from https://develop.svn.wordpress.org/trunk@52074


git-svn-id: http://core.svn.wordpress.org/trunk@51666 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/user.php

@@ -3110,7 +3110,14 @@ function register_new_user( $user_login, $user_email ) {
 		$errors->add( 'invalid_email', __( '<strong>Error</strong>: The email address isn&#8217;t correct.' ) );
 		$user_email = '';
 	} elseif ( email_exists( $user_email ) ) {
-		$errors->add( 'email_exists', __( '<strong>Error</strong>: This email is already registered. Please choose another one.' ) );
+		$errors->add(
+			'email_exists',
+			sprintf(
+				/* translators: %s: Link to the login page. */
+				__( '<strong>Error:</strong> This email address is already registered. <a href="%s">Log in</a> with this address or choose another one.' ),
+				wp_login_url()
+			)
+		);
 	}
 
 	/**