cmd/go: suggest running 'go help gopath' with problematic GOPATH

cmd/go: suggest running 'go help gopath' with problematic GOPATH

Suggest running 'go help gopath' when the running 'go install .'
and the folder is outside of GOPATH.

Added link to 'https://golang.org/doc/code.html' in gopath help
for more information.

Example output:

% go install .
go install: no install location for directory f:\x\badmessage outside GOPATH
        please run 'go help gopath' for more information

% go help gopath
... SNIP ...
See https://golang.org/doc/code.html for an example.

Fixes #8457

Change-Id: I0ef6ee3c65bb12af2168eafeb757258aa3835664
Reviewed-on: https://go-review.googlesource.com/9258
Reviewed-by: Russ Cox <rsc@golang.org>