cmd/go/internal/modload: handle NotExist errors in (*mvsReqs).Previous

Previous is used during downgrading. If the module proxy does not
advertise any versions (for example, because it contains only
pseudo-versions), then Previous should return "none" instead of a
non-nil error.

For #37438

Change-Id: I4edfec19cfeb3ffe50df4979f99a01321c442509
Reviewed-on: https://go-review.googlesource.com/c/go/+/266370


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