cmd/vet: return nil from importType if it fails.
cmd/vet: return nil from importType if it fails.

This permits vet to not panic during init if fmt.a is unavailable.

LGTM=r
R=gri, r
CC=golang-codereviews
https://codereview.appspot.com/153590043

karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools,karmakaze/go.tools