Improve error messages

[dstrockis]

While playing with the lib I encountered less-than-helpful error messages a few times. I didn't write all of them down, but here's one example I remember:

"Error: JWK kid does not match Did publickey id"

This was not very useful to me, because I wasn't sure which value was which. It would be better if the error message said:

"Error: JWK kid 'foo' does not match Did publickey id 'bar'"

I'll add more examples to this list as I remember them.