Try to connect in Begin() too. Postpone error handling to Interceptor.

This change will improve Mongo and Revel plays nicer together in case Mongo is not started yet or restarted while Revel is running.

Dial() error is postponed until the Before() interceptor and Revel won't panic any longer.

This will also prevent the panic (which prints an non-userfriendly stack trace) and instead log (or in dev mode display) an easy to understand error message.