Add language for handling implementation mismatches

[decentralgabe]

What if a credential uses JSON Schema Draft 7, enabled by this specification, but an implementer who sees such a VC only supports draft 2020-12?

Let's add some language on what should practically occur during implementation gaps.

[decentralgabe]

My gut is to say "validation failed" but a binary is not really good enough. Three statuses would be preferable, but that may be adding too much complexity to the spec. Keeping a binary pass/fail could work with a reason field as well.

I am wondering if there's an analog to be made to checking a credential's status but the status list cannot be resolved for some reason. Is this behavior just undefined? What guidance is there for verifiers?

Curious on other's thoughts: @msporny @OR13