Don't swallow errors in :unsuball

More code that should get fixed if we ever use GError or some other saner error
reporting mechanism. For now, I suppose this mess with the lower-level function
reporting error and the higher-level function reporting success is in good
company.

Test by doing a kdestroy before :unsuball. It should complain about a SERVNAK.
(It really should complain about missing credentials cache, but apparently
libzephyr tries again without auth in ZCancelSubscription even though the
server rejects unauthenticated ones. Meh.)