go/analysis/unitchecker: improve error message for go tool vet
go/analysis/unitchecker: improve error message for go tool vet

Previously running "go tool vet" said

vet: invalid command: want .cfg file (this reduced version of vet is
intended to be run only by the 'go vet' command)

With this change it says:

vet: invoking "go tool vet" directly is unsupported; use "go vet"

Updates golang/go#28869

Change-Id: I603ab2f75bb52d860e5cd7466e89d051dfbf3f08
Reviewed-on: https://go-review.googlesource.com/c/152217
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Alan Donovan <7069e9552f9d110b0a0f326b96f22f63fb99dfc1@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools