Standardizing error messages for Rekor API incompatibility

[haydentherapper]

Description

Whenever a client that only supports Rekor v1 entries tries to verify a Rekor v2 entry, it will fail. At the very least, it must fail gracefully with a meaningful error, e.g. "unsupported type", rather than an unmarshalling error. Ideally a client should also signify that a user should upgrade to the latest version of the client.

[jku]

We've discussed this (requiring specific error messages) with @loosebazooka at some point. At least I was not completely convinced this would be ultimately beneficial...

It sounds like any addition or change in the expected messages will result in required changes in N-1 clients (with the one client being the one whose error message was chosen as the blessed one).