Add missing lang string for warning (Bug #1227372)

Lang string was missing for "Allowed iframe sources"

Change-Id: I1cab612134a334fac7f0ca54fcf406b4a08f3226
Signed-off-by: Kristina D.C. Hoeppner <kristina@catalyst.net.nz>

htdocs/admin/extensions/iframesites.php

@@ -196,7 +196,7 @@ function newurl_validate(Pieform $form, $values) {
         $form->set_error('url', get_string('iframeinvalidsite', 'admin'));
     }
     if (isset($iframesources[$urldata['key']])) {
-        $form->set_error('url', get_string('urlalreadyexists'));
+        $form->set_error('url', get_string('urlalreadyexists', 'admin'));
     }
 }
 

htdocs/lang/en.utf8/admin.php

@@ -155,6 +155,7 @@ $string['iframeurldescription'] = "Text to match at the beginning of the iframe
 $string['iframedisplaynamedescription'] = 'The name of the site to be displayed to users.';
 $string['iframeinvalidsite'] = "This field should contain a valid host and an optional path. It can contain only letters, digits, '.', '/', '_', and '-'.";
 $string['iframeiconhost'] = 'Icon host';
+$string['urlalreadyexists'] = 'This URL already exists. You cannot add it twice.';
 $string['iframeiconhostdescription'] = 'If you wish, you may specify a different host for the favicon image. All sites with the same name will use this icon.';
 
 $string['cleanurls'] = 'Clean URLs';