Remove unnecessary !isValid check, add reason to verificationResult

[DJHunn39]

There's an old FIXME in place here. The PR it refers to has been merged, so it can be removed.

It would also be helpful to include a reason property in the object returned by a PresentationVerificationCallback function, such that it will be included in the message of the error thrown by the Sphereon library here. This is, in a way, related to #144, but the changes required to just supply a reason to OID4VC can be made in @credo-ts.

These adjustments were discussed with @TimoGlastra in a meeting, though that was some time ago.

I have a branch with proposed changes ready to go, I'll open a PR shortly.