While setting up a Chromium repo on a new machine I ended up with a
missing .gclient_entries file. This triggered a warning message but it
led me in the wrong direction. I think that the new message - saying
exactly what problem was detected - will be slightly more helpful.

Change-Id: I8c91861f843e3d30881b1b4933a02ad966c114ef
Reviewed-on: https://chromium-review.googlesource.com/533314Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>