openpgp: Raise error conditions instead of calling 'error'.

* guix/openpgp.scm (&openpgp-error, &openpgp-unrecognized-packet-error)
(&openpgp-invalid-signature-error): New error conditions.
(openpgp-hash-algorithm): Add 'signature-port' parameter.  Raise an
error condition instead of calling 'error'.
(parse-subpackets): Likewise.
(get-data): Raise instead of calling 'error'.
(get-openpgp-detached-signature/ascii): Likewise.
(get-signature): Likewise.