cmd/go/internal/cfg: improve error message when GOPATH is unset
Add GoPathError variable. This variable gets set when GOPATH is not
set in the environment and in build.Default.GOPATH. GoPathError may
be used in "GOPATH unset" error messages to explain why GOPATH is not
set.
This CL improves upon CL 158257.
Fixes #29341
Change-Id: Ib42b42fb442c8297d58da4ca556be55e21a034e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/331529 Reviewed-by: Bryan C. Mills <bcmills@google.com> Reviewed-by: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Trust: Michael Knyszek <mknyszek@google.com>