Give a better error message for external shares with self-signed ssl certificates

apps/files_sharing/ajax/external.php

@@ -33,6 +33,11 @@ $externalManager = new \OCA\Files_Sharing\External\Manager(
 
 $name = OCP\Files::buildNotExistingFileName('/', $name);
 
+// check for ssl cert
+if (substr($remote, 0, 5) === 'https' and !OC_Util::getUrlContent($remote)) {
+	\OCP\JSON::error(array('data' => array('message' => $l->t("Invalid or untrusted ssl certificate"))));
+	exit;
+} else {
 	$mount = $externalManager->addShare($remote, $token, $password, $name, $owner);
 	/**
 	 * @var \OCA\Files_Sharing\External\Storage $storage
@@ -46,3 +51,4 @@ if($result){
 		$externalManager->removeShare($mount->getMountPoint());
 		\OCP\JSON::error(array('data' => array('message' => $l->t("Couldn't add remote share"))));
 	}
+}