Revert of Revert "gclient: Change the .gclient URL mismatch warning into an error." (https://codereview.chromium.org/213483003/)

Reason for revert:
Re-landing this after https://codereview.chromium.org/214813002/

Original issue's description:
> Revert "gclient: Change the .gclient URL mismatch warning into an error."
> 
> This reverts commit c01bff75.
> 
> BUG=356881
> TBR=iannucci
> 
> Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=259727

TBR=iannucci@chromium.org,smut@google.com
NOTREECHECKS=true
NOTRY=true
BUG=356881

Review URL: https://codereview.chromium.org/221423004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@261158 0039d316-1c4b-4281-b951-d872f2087c98