PRESUBMIT: Add check for checkdeps.

Several times I've run into the problem with
presubmit crashing when uploading a CL from a checkout
where gclient sync hasn't run yet.
This will print a user friendly error message instead.

BUG=
R=phoglund@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/32699004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7824 4adac7df-926f-26a2-2b94-8c16560cd09d