Add ServiceWorker error enums

Flushing out actual error conditions that can be thrown by the backend.

BUG=
TBR=abarth@chromium.org

Review URL: https://codereview.chromium.org/107293009

git-svn-id: svn://svn.chromium.org/blink/trunk@163811 bbb929c8-8fbe-4397-9dbb-9b2b20218538