backup: do not show full stacktrace to the user
backup: do not show full stacktrace to the user

Opt for a simple one-liner error messages, instead of meaningless stack
trace (it's most of the time about qubesd responding with error, so the
stack trace of actual problem is elsewhere).

marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client,marmarek/qubes-core-mgmt-client