[release-branch.go1.11] cmd/go: don't fail if requested Go version is later than current one

This is a partial backport of CL 147278 from tip to the Go 1.11 branch.

Change the behavior when the go.mod file requests a Go version that is
later than the current one. Previously cmd/go would give a fatal error
in this situation. With this change it attempts the compilation, and
if (and only if) the compilation fails it adds a note saying that the
requested Go version is newer than the known version.  This is as
described in https://golang.org/issue/28221.

Updates #28221

Change-Id: Iea03ca574b6b1a046655f2bb2e554126f877fb66
Reviewed-on: https://go-review.googlesource.com/c/151358


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>