mnet MDL-21692 print an error when we can't contact the host when adding a new peer

lang/en_utf8/mnet.php

@@ -301,4 +301,5 @@ $string['importfields'] = 'Fields to import';
 $string['exportfields'] = 'Fields to export';
 $string['defaultfields'] = 'The globally set fields, which you are about to override, are: ';
 $string['leavedefault'] = 'Use the default settings instead';
+$string['noaddressforhost'] = 'Sorry, but that hostname ($a) could not be resolved!';
 ?>

mnet/peer.php

@@ -58,8 +58,7 @@ class mnet_peer {
 
         // Couldn't find the IP address?
         if ($ip_address === $hostname && !preg_match('/^\d+\.\d+\.\d+.\d+$/',$hostname)) {
-            $this->error[] = array('code' => 2, 'text' => get_string("noaddressforhost", 'mnet'));
-            return false;
+            throw new moodle_exception('noaddressforhost', 'mnet', '', $hostname);
         }
 
         $this->name = $wwwroot;