Running gclient validate or gclient flatten outside of gclient root
results in an unhelpful AttributeError stacktrace. This change adds an
error message that lets the user know the client is not confugred.

Bug: 732733
Change-Id: I33b1a08aa394145e238a4939bfd995ec33a8df17
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2624351Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>