lib/gpg: Add more specific OstreeGpgError codes

lib/gpg: Add more specific OstreeGpgError codes

Currently `ostree_gpg_verify_result_require_valid_signature` always
returns an error that the key used for the signature is missing from the
keyring. However, all that's been determined is that there are no valid
signatures. The error could also be from an expired signature, an
expired key, a revoked key or an invalid signature.

Provide values for these missing errors and return them from
`ostree_gpg_verify_result_require_valid_signature`. The description of
each result is appended to the error message, but since the result can
contain more than one signature but only a single error can be returned,
the status of the last signature is used for the error code. See the
comment for rationale.

Related: flatpak/flatpak#1450