Use error messages to be consistent, and avoid checking for a returned
value when dbus-glib function to fetch it returned false, as it's
implicit that we had a failure.
Otherwise if didn't fail we are sure that we got the requested value.
If we get a `NoEnrolledPrints` error while list, we don't consider it an
hard error and in such case we proceed to releasing the device, but without
clearing the previously set error first.
These utilities are generally useful beyond only testing purproses. And,
since it is desirable to have automated tests inside the tests
subdirecty, it makes sense to move them elsewhere.
