As a n0oB to Chrome, I sometimes deleted uploaded CLs on codereview.chromium.org
after looking at them and deciding they were unsuitable for review. I
would then later git cl upload and get a confusing stacktrace about a
404. This change lets the user know why the error happened, and how to
resolve it.

I decided not to auto-scrub the old issue on the client side, because
the 404 might be spurious (server misconfiguration, bug in my code, etc.).
Safer to make the user explicitly run the command.

Review URL: http://codereview.chromium.org/9091009

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@116360 0039d316-1c4b-4281-b951-d872f2087c98