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
In core-admin matching collections are real dicts, so clone this API
behaviour here too. Specific changes:
- iteration yields keys, not values
- implement values and items methods
Additionally fix keys method, it was broken on python2 (list have no
copy method).
