backup/restore - better error msg when user/pswd was needed

From Perry...

> -Is it required to supply the administrator username/pass?

If you're just doing the default bucket, no user/pswd is needed.

But user/pswd is needed for multiple buckets.

> Can we prompt for a user/pass if none is given?

I'd rather not and instead give some better error message, perhaps
with an example. I'll make a better error message.