Display a warning prompt in presubmit if the tree status is in 'Caution'.
Display a warning prompt in presubmit if the tree status is in 'Caution'.

Example presubmit output:


Running presubmit upload checks ...

** Presubmit Warnings **
Tree state is: caution

***************
Please use caution in checkins
http://skia-tree-status-staging.appspot.com/banner-status?format=json
***************

There were presubmit warnings. Are you sure you wish to continue? (y/N):
Review URL: https://codereview.appspot.com/7200043

git-svn-id: e8541e15acce502a64c929015570ad1648e548cd@7337 2bbb7eff-a529-9590-31e7-b0007b416f81

Cue/skia,Cue/skia,Cue/skia,Cue/skia