Handle resource fetch failure in TextTrackLoader::load

It's not clear when this can happen, even after looking at
ResourceFetcher::requestResource I was unable to produce a test
case which would hit this code path. However, given that we are
null-checking already, we might as well handle it correctly.

BUG=313287

Review URL: https://codereview.chromium.org/53213002

git-svn-id: svn://svn.chromium.org/blink/trunk@161040 bbb929c8-8fbe-4397-9dbb-9b2b20218538