Add an Error class for ServiceWorkers

This class could eventually be generically exposed as WebError, and wrap DOMError.

BUG=285976

Review URL: https://codereview.chromium.org/27278002

git-svn-id: svn://svn.chromium.org/blink/trunk@161275 bbb929c8-8fbe-4397-9dbb-9b2b20218538