getSVNInfo() is expected to fail in some use case. Make this behavior explicit.

R=dpranke@chromium.org
BUG=
TEST=

Review URL: http://codereview.chromium.org/6881008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@81970 0039d316-1c4b-4281-b951-d872f2087c98