cmd/compile/internal/types2: review of errors.go

The changes between (equivalent, and reviewed) go/types/errors.go
and errors.go can be seen by comparing patchset 1 and 2. The actual
changes are removing the "// UNREVIEWED" marker.

The go/types version is significantly different as it handles
error codes but doesn't have some of the types2 changes.

Change-Id: I48f79ce31490db938c830df7d38e247d55d54f2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/305577


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>