chore(404): Extend 404 error message to mention possibly missing permissions

Signed-off-by: Marcel Klehr <mklehr@gmx.net>

core/templates/404.php

@@ -18,7 +18,7 @@ if (!isset($_)) {//standalone  page is not supported anymore - redirect to /
 	<div class="body-login-container update">
 		<div class="icon-big icon-search"></div>
 		<h2><?php p($l->t('Page not found')); ?></h2>
-		<p class="infogroup"><?php p($l->t('The page could not be found on the server.')); ?></p>
+		<p class="infogroup"><?php p($l->t('The page could not be found on the server or you may not be allowed to view it.')); ?></p>
 		<p><a class="button primary" href="<?php p(\OC::$server->getURLGenerator()->linkTo('', 'index.php')) ?>">
 			<?php p($l->t('Back to %s', [$theme->getName()])); ?>
 		</a></p>