cmd/go/internal/modfetch: use errors.ErrUnsupported

cmd/go/internal/modfetch: use errors.ErrUnsupported

CL 473935 added errors.ErrUnsupported, let's use it.

Updates #41198

Change-Id: If6534d19cb31ca979ff00d529bd6bdfc964a616d
Reviewed-on: https://go-review.googlesource.com/c/go/+/476135
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>