cmd/vet: don't exit with failure on type checking error

The vet tool only reports a type checking error when invoked with -v.
Don't let that by itself cause vet to exit with an error exit status.

Updates #21188

Change-Id: I172c13d46c35d49e229e96e833683d8c82a77de7
Reviewed-on: https://go-review.googlesource.com/52851


Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Rob Pike <r@golang.org>