`sop verify` should be able to report unmatched signatures to a file

One concern about about verifying signatures over time, for example, for a software distribution, is to be able to tell when some signatures are not considered valid for some reason by the OpenPGP verifier, but you're still seeing them.

For example, if you distribute a new key and the old implementations don't know how to parse the signature, you might want to know how many old implementations that don't parse that signature there are, especially if there are valid signatures on the same data file.

As the verifier for a distro, you could configure your tooling around sop to produce a structured report back to your issue tracker or other system if there is anything in this channel.

Proposed syntax for new argument:

sop verify --bad-signatures-out=SIGNATURES

and

sop inline-verify --bad-signatures-out=SIGNATURES

Maybe this is always ASCII-armored output, and we could encourage an implementation that rejects a signature in this way to mention in the Comment: field in the ASCII armor why the various signatures included failed to validate.

We could also add a --no-armor argument, like sop inline-detach, which explicitly affects this output, but not stdout. If --no-armor is supplied, perhaps an implementation could store the human-readable explanations for sig failure in a custom notation. (or maybe no human-readable notation is produced for non-armored outputs).

Would we also want to add such an option to sop decrypt?