Improve gclient_scm SVN deprecation message.
Improve gclient_scm SVN deprecation message.

R=hinoka@chromium.org, maruel@chromium.org, stip@chromium.org
BUG=513567,475230

Review URL: https://codereview.chromium.org/1260913002

git-svn-id: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@296138 0039d316-1c4b-4281-b951-d872f2087c98

primiano/depot_tools,disigma/depot_tools,duanwujie/depot_tools,azunite/chrome_build,hsharsha/depot_tools,cpanelli/-git-clone-https-chromium.googlesource.com-chromium-tools-depot_tools,hsharsha/depot_tools,CoherentLabs/depot_tools,mlufei/depot_tools,aleonliao/depot_tools,azureplus/chromium_depot_tools,kaiix/depot_tools,ajohnson23/depot_tools,aleonliao/depot_tools,Midrya/chromium,kaiix/depot_tools,cpanelli/-git-clone-https-chromium.googlesource.com-chromium-tools-depot_tools,mlufei/depot_tools,azureplus/chromium_depot_tools,disigma/depot_tools,Midrya/chromium,disigma/depot_tools,kaiix/depot_tools,hsharsha/depot_tools,mlufei/depot_tools,duongbaoduy/gtools,Midrya/chromium,ajohnson23/depot_tools,azunite/chrome_build,CoherentLabs/depot_tools,azunite/chrome_build,aleonliao/depot_tools,duanwujie/depot_tools,azureplus/chromium_depot_tools,duanwujie/depot_tools,primiano/depot_tools,ajohnson23/depot_tools,duongbaoduy/gtools,primiano/depot_tools,duongbaoduy/gtools,cpanelli/-git-clone-https-chromium.googlesource.com-chromium-tools-depot_tools