Simplify GIT.Capture() code to always redirect stderr by default and always throw an exception on failure.

Make gclient_scm_test silent.

Replace raise Exception() with raise gclient_utils.Error().

BUG=54084
TEST=none

Review URL: http://codereview.chromium.org/3353018

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@58936 0039d316-1c4b-4281-b951-d872f2087c98