This change reworks the sentinel error value ErrNoModRoot and the type
noMainModulesError so that we can determine the appropriate error
message to display based on the loader state without depending on the
state directly.
This commit is part of the overall effort to eliminate global
modloader state.
Change-Id: I64de433faca96ed90fad4c153766c50575a72157
Reviewed-on: https://go-review.googlesource.com/c/go/+/711120 Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Matloob <matloob@golang.org>