Clarify an empty object in a VP Token cannot be used to signify an error response

[awoie]

Fixes #743

• needs errata? change 1.0?

I also improved the vp_token definition because it is a quite important part of the spec and the readability was not great.

I removed this editorially because I don't think it had any meaning anymore:

The same rules as above apply for encoding the Presentations.

[awoie]

We probably will need to apply this logic to DC API as well. I will need to double check if this is included with the current proposed text.

[fkj]

If people don't object to the rewriting of the definition, I agree that this makes it easier to understand what we actually mean.