cmd/go: display helpful error when module cache can't be created

Previously when the module cache specified by GOMODCACHE could not be
created an unhelpful message would be printed multiple times.

This happened because we were fetching several things in parallel then
failing to write them because we can't create the module cache.

We now check if the module cache can be created before fetching.

If not, the following message is printed:

go: could not create module cache

Fixes #45113

Change-Id: Ic9cec787411335edc7f4d0614fde7eaa8a957fb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/304571


Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>