Handle ERRSIG in the gpgv method like BADSIG

Handle ERRSIG in the gpgv method like BADSIG

ERRSIG is created whenever a key uses an unknown/weak digest
algorithm, for example. This allows us to report a more useful
error than just "unknown apt-key error.":

The following signatures were invalid: ERRSIG 13B00F1FD2C19886 1 2 01 1457609403 5

While still not being the best reportable error message, it's
better than unknown apt-key error and hopefully redirects users
to complain to their repository owners.