token: more descriptive panics

Currently, there are several panics in token that simply say "illegal!". This CL
adds the values.

This is valuable when the token call is wrapped under several layers and you
can't easily see which value is being passed to token.

Change-Id: Ib04b55cafcd9b9ec6820dcf416fc4d49afaea15f
Reviewed-on: https://go-review.googlesource.com/c/go/+/262017


Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Trust: Robert Griesemer <gri@golang.org>
Trust: Jean de Klerk <deklerk@google.com>
TryBot-Result: Go Bot <gobot@golang.org>