[release-branch.go1.11] cmd/go: don't panic when go run is passed ... under nonexistent dir

Given a nonexistent directory above a wildcard:

    go run ./nonexistent/...

Print this error instead of panicking:

    go run: no packages loaded from ./nonexistent/...

Updates #28696.
Fixes #28725

Change-Id: Iaa3bc5c78b14ef858d931778e1bc55ca626c5571
GitHub-Last-Rev: bb1a80483ad26c8cf646cf0900d08cfe49aba535
GitHub-Pull-Request: golang/go#28703
Reviewed-on: https://go-review.googlesource.com/c/148821


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
(cherry picked from commit 529ea7c0)
Reviewed-on: https://go-review.googlesource.com/c/149607
Run-TryBot: Ian Lance Taylor <iant@golang.org>