cmd/compile/internal/types2: change Checker.verify to return an error

In preparation for upcoming API changes, change the internal API for
verification of type arguments to return an error and argument index,
and use this to lift up error reporting into Instantiate.

Change-Id: I88b1e64dd9055c4c20c0db49c96c79c5da894450
Reviewed-on: https://go-review.googlesource.com/c/go/+/342151


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>