cmd/go/internal/modload: improve ErrNoModRoot error text
Before this change, in several cases where HasModRoot() returned false,
we'd return ErrNoModRoot. ErrNoModRoot would say that there was no
go.mod file but would not mention workspaces. With this change,
ErrNoModRoot will return error text that's different if we're in a
workspace, saying that there are no modules in the workspace.
Fixes #54419
Change-Id: I77c94d0011947bf8e33c066416ab3762502fd2e6
Reviewed-on: https://go-review.googlesource.com/c/go/+/660675
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com>