Better error message when json isn't a verifiable credential

[jchartrand]

When JSON that isn't a verifiable credential is pasted into VerifierPlus, it displays the following error:

The error is misleading because it is shown even if the JSON can be parsed. It seems to be a catch-all error for various things including the case where the JSON is not a VC, as in the above example where the JSON was actually an OBv2 (so not a verifiable credential).

Probably want a message more like: "The JSON is not a Verifiable Credential."

And maybe even more specifically - when an OBv2 is pasted in: "This is an OpenBadge V2 which is not a Verifiable Credential. Version 3 of OpenBadges are Verifiable Credentials."