cmd/go: improve error message from '-mod=readonly'

[leitzler]

What version of Go are you using (go version)?

$ go version
go version go1.11.2 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
N/A

What did you do?

Run go build -mod=readonly when an update of go.mod is needed.

What did you expect to see?

A more descriptive error message. Since -mod=readonly is typically supposed to be used in CI, it will save a lot of manual work if the error message did actually tell why an update to go.mod is needed.

E.g.:
go: github.com/sirupsen/logrus@v1.2.0 requires github.com/stretchr/testify@v1.2.2, but go.mod require @v1.2.1 and updates is disabled by -mod=readonly

What did you see instead?

go: updates to go.mod needed, disabled by -mod=readonly

[myitcv]

cc @bcmills

[bcmills]

Would #27005 address your use-case? Presumably that command would list all of the updates needed on failure.

[leitzler]

As a workaround, probably.

But since the tool already knows what updates are needed, there is no reason to do it in two steps afaik.
That would require the user to have knowledge of how modules works and what command to run to get the detailed output.