cmd/go: go mod why exit status should be non-zero if module is not used

[rogpeppe]

$ go version
go version devel +ce7534ff06 Fri Mar 8 13:46:43 2019 +0000 linux/amd64

When go mod why is asked to show a module that isn't a dependency, it prints a "main module does not need" line, but it succeeds regardless.

Perhaps it should be changed so that the exit status reflects whether the dependency was actually found or not.

[bcmills]

See also #27900.

[mvdan]

This would have made a CI script quite a lot simpler for me. The closest I can do right now is something like go mod why <arg> | grep 'does not need'.