The code assumes that QubesVMInUseError is thrown for a failed
dependency check, but there are also other reasons. We need to
handle the case when dependency list is empty, and also display
the original message.
See also QubesOS/qubes-core-admin#308 for which this was needed.
QubesException class is used with meaningful messages and should be ok
to use it directly as error message. For other exceptions, still use
full traceback (most likely a bug somewhere, not user error).
FixesQubesOS/qubes-issues#3610
