sigstore, test: drastically simplify error types

[woodruffw]

WIP.

Another chore/refactor PR: this drastically reduces the number of error types we have floating around the codebase, in favor of a single unified VerificationError (which we can specialize in the future, as necessary).

Additionally, this changes the verify(...) API from returning a VerificationResult to using exceptional error handling. There are three justifications for this:

1. It allows us to remove the VerificationResult model and its subclasses, which are mostly just duplication over existing error types
2. It reduces the likelyhood of VerificationErrors leaking through the API accidentally
3. It aligns the API with that the DSSE/in-toto API will do, which is use a return value on the success case and exceptions for all errors.

In the process, this also brings our coverage up a bit.

[jku]

Nice,

• Totally agree about the exception simplification
• Verifier.verify() API change is a little annoying but it does seem to make a lot of code more straight forward

left a question on the argument parsing, otherwise LGTM.