go/types: cleanup panic calls

go/types: cleanup panic calls

This is a port of CL 339969 to go/types. It differs slightly in
errors.go, due to the differing API.

Change-Id: Ie2bf84ebf312ea3872ee6706615dfc6169a32405
Reviewed-on: https://go-review.googlesource.com/c/go/+/342431
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>