mailer: fix error handling

previous mailer code has no callback and thus no way to pass back errors.
now with asyncification it passes back the error

src/docker.js

@@ -30,7 +30,7 @@ exports = module.exports = {
     createVolume,
     removeVolume,
     clearVolume,
-    update
+    update,
 };
 
 const apps = require('./apps.js'),