Don't throw an error if not on stabilizing branch and give more information on exit.
Don't throw an error if not on stabilizing branch and give more information on exit.

TEST=Ran "cros_mark_as_stable push" with no stabilizing_branch

Review URL: http://codereview.chromium.org/3159010

Change-Id: I31dadc354e224f0f19f0878581204678391ac108

mjg59/scripts,BugRoger/scripts,BugRoger/scripts,sigma/coreos-scripts,trnubo/scripts,sigma/coreos-scripts,mjg59/scripts,trnubo/scripts,vmware/coreos-scripts,fivethreeo/scripts,nicescale/scripts,bcwaldon/coreos-scripts,endocode/scripts,coreos/scripts,endocode/scripts,smilart/scripts,trnubo/scripts,bcwaldon/coreos-scripts,gtank/scripts,BugRoger/scripts,smilart/scripts,zhang0137/scripts,gtank/scripts,fivethreeo/scripts,andrejro/coreos--scripts,marineam/coreos-scripts,gtank/scripts,fivethreeo/scripts,zhang0137/scripts,vmware/coreos-scripts,endocode/scripts,crawford/scripts,marineam/coreos-scripts,nicescale/scripts,andrejro/coreos--scripts,crawford/scripts,marineam/coreos-scripts,andrejro/coreos--scripts,coreos/scripts,mischief/scripts,mischief/scripts,glevand/coreos--scripts,glevand/coreos--scripts,mischief/scripts,vmware/coreos-scripts,bcwaldon/coreos-scripts,crawford/scripts,smilart/scripts,nicescale/scripts,mjg59/scripts,sigma/coreos-scripts,zhang0137/scripts