Get back detailed results on which signatures are present vs. not present, match vs mismatch

[dralley]

The functions which verify digests and signatures ought to provide some way to inspect the results of the verification in a more detailed way.

e.g. something like

struct DigestReport {
     payloaddigest: DigestStatus,
     sha256header: DigestStatus,
     sha1header: DigestStatus,
     // etc.
}

enum DigestStatus {
    Success,
    NotPresent,
    Mismatch(String, String),
}

where the report can be easily collapsed into a single Result<...: RPMError> for success or failure