cmd/go/internal/modload: in importFromModules, do not wrap module graph errors...

cmd/go/internal/modload: in importFromModules, do not wrap module graph errors in ImportMissingError

If an error occurs in loading the module graph (such as a missing
checksum for a relevant go.mod file), that error should be terminal
and we should not look elsewhere to try to resolve the import. An
ImportMissingError instructs the caller to do exactly that, so don't
use that error type for this case.

(This behavior is tested incidentally in a later CL in this stack.)

For #36460

Change-Id: I963e39cc7fbc457c12a626c1402c0be29203d23b
Reviewed-on: https://go-review.googlesource.com/c/go/+/314633


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>