uscan: Inability to verify GPG signature is a fatal error

If uscan successfully downloads an archive and is configured to verify
the GPG signature, then any failure to do so will cause uscan to exit
with an error.  This includes not having the keyring, inability to
download the detached signature, and the actual gpg(v) verification
failing.

Signed-off-by: James McCoy <jamessan@debian.org>