Improve --help and usage message for roll_deps.py
Improve --help and usage message for roll_deps.py

Also, don't use assert for user error handling plus
some other general python cleanup.

NOPRESUBMIT=true (presubmit is broken)

Review URL: https://codereview.chromium.org/1091603002

git-svn-id: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@294950 0039d316-1c4b-4281-b951-d872f2087c98

aleonliao/depot_tools,disigma/depot_tools,chinmaygarde/depot_tools,primiano/depot_tools,duongbaoduy/gtools,duongbaoduy/gtools,SuYiling/chrome_depot_tools,Midrya/chromium,azunite/chrome_build,hsharsha/depot_tools,aleonliao/depot_tools,ajohnson23/depot_tools,ajohnson23/depot_tools,SuYiling/chrome_depot_tools,liaorubei/depot_tools,liaorubei/depot_tools,ajohnson23/depot_tools,cpanelli/-git-clone-https-chromium.googlesource.com-chromium-tools-depot_tools,chinmaygarde/depot_tools,withtone/depot_tools,CoherentLabs/depot_tools,liaorubei/depot_tools,gcodetogit/depot_tools,SuYiling/chrome_depot_tools,Midrya/chromium,CoherentLabs/depot_tools,withtone/depot_tools,npe9/depot_tools,duanwujie/depot_tools,fracting/depot_tools,npe9/depot_tools,azunite/chrome_build,duongbaoduy/gtools,kaiix/depot_tools,gcodetogit/depot_tools,kaiix/depot_tools,disigma/depot_tools,npe9/depot_tools,disigma/depot_tools,azunite/chrome_build,liaorubei/depot_tools,hsharsha/depot_tools,mlufei/depot_tools,mlufei/depot_tools,azureplus/chromium_depot_tools,duanwujie/depot_tools,azureplus/chromium_depot_tools,azureplus/chromium_depot_tools,fracting/depot_tools,primiano/depot_tools,Midrya/chromium,gcodetogit/depot_tools,kaiix/depot_tools,withtone/depot_tools,cpanelli/-git-clone-https-chromium.googlesource.com-chromium-tools-depot_tools,fracting/depot_tools,chinmaygarde/depot_tools,cpanelli/-git-clone-https-chromium.googlesource.com-chromium-tools-depot_tools,mlufei/depot_tools,primiano/depot_tools,duanwujie/depot_tools,npe9/depot_tools,aleonliao/depot_tools,hsharsha/depot_tools