Added try/except to the places where errors are tolerated.

Renamed scm.SVN.CaptureBaseRevision to CaptureRevision and removed CaptureHeadRevision.

BUG=54084
TEST=none

Review URL: http://codereview.chromium.org/3307016

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@58932 0039d316-1c4b-4281-b951-d872f2087c98