gnupg: 'gnupg-verify*' returns a status symbol.

This allows callers to distinguish between signature verification
failure and missing key.

* guix/gnupg.scm (gnupg-receive-keys): Return true on success.
(gnupg-verify*): Check return value of 'gnupg-receive-keys'.  Return two
values, the first one being a symbol.
* guix/upstream.scm (download-tarball): Get the two return values of
'gnupg-verify*', and match on the first one.
* gnu/packages/bash.scm (download-patches): Check the first return value
of 'gnupg-verify*'.

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