QubesVM.run_for_stdio() by default captures stderr. In case of call fail
(non-zero return code), captured stderr is included in the exception
object, but isn't printed by default CalledProcessError message.
Make it visible by:
- handling CalledProcessError and including in the test failure message
(when exception is captured already)
- not capturing stderr (if no exception handling is present in the
test)
5423ead27a