MDL-50824 message/externallib.php: Fix get_string call

This fixes the get_string call by adding the intended recipient's
fullname as the third argument when a user tries to message the
recipient, the recipient is blocking non-contacts from messaging
them, and the user is not in the recipient's contact list.

message/externallib.php

@@ -137,7 +137,8 @@ public static function send_instant_messages($messages = array()) {
             if ($success && empty($contactlist[$message['touserid']]) && !empty($blocknoncontacts)) {
                 // The user isn't a contact and they have selected to block non contacts so this message won't be sent.
                 $success = false;
-                $errormessage = get_string('userisblockingyounoncontact', 'message');
+                $errormessage = get_string('userisblockingyounoncontact', 'message',
+                        fullname(core_user::get_user($message['touserid'])));
             }
 
             //now we can send the message (at least try)