CLOUDSTACK-4698: Check DHCP service in the network before get dhcp service provider
Apply https://reviews.apache.org/r/14084/
CLOUDSTACK-4688: UI > (1) Notifications widget - pollTimer() - error handling - check if args is null before trying to access args.message property (2) sharedFunctions.js - pollAsyncJobResult() - error handling - pass message argument to args.error().