cmd/go: improve error message when outside a module from "working directory is not part of a module" [1.16 backport] #44746
Description
@jayconrod requested issue #44745 (fixed in b87e9b9 and b62da08 for Go 1.17) to be considered for backport to the next 1.16 minor release.
@gopherbot Please backport to Go 1.16. This message was triggered by #41330, and it's unexpected by most. Improving the error message carries little risk.