cmd/go/internal/modfetch: distinguish "unsupported" errors from RecentTag

cmd/go/internal/modfetch: distinguish "unsupported" errors from RecentTag

CL 426079 started checking errors from RecentTag.
Unfortunately, we forgot to run "-longtest" SlowBots, and it turns out
to have broken non-short tests for non-git VCS implementations,
because those don't implement the RecentTag method.

Updates #53935.

Change-Id: I5935f2f4b3f684515e99e8bf70a840154c36249f
Reviewed-on: https://go-review.googlesource.com/c/go/+/426495
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>