Change ThreadSafeCookieJar to not crash on invalid path.
Change ThreadSafeCookieJar to not crash on invalid path.

The Windows Swarm slaves have trouble exanding '~' so in the meantime, make
them not crash if the file path is invalid.

TBR=csharp@chromium.org
BUG=


Review URL: https://chromiumcodereview.appspot.com/13682002

git-svn-id: d5a9b8648c52d490875de6588c8ee7ca688f9ed1@192430 0039d316-1c4b-4281-b951-d872f2087c98

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py