go/types: change Checker.verify to return an error
This is a port of CL 342151 to go/types, adjusted for errors and
positions. Checker.sprintf was refactored to facilitate formatting
error messages with a nil Checker.
Change-Id: Ib2e5c942e55edaff7b5e77cf68a72bad70fea0b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/342670
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>