Fix potential crash causes in ServiceWorker LayoutTests

1. Adds an empty serviceworker.js
Registering nonexistent worker script will start to return InstallError,
so create an empty serviceworker.js in the LayoutTests (we should also
add nonexistent worker case once the registration patch re-lands)

2. Handle all possible error cases in ServiceWorkerError.cpp
Getting an error other than DisabledError hits ASSERT in debug build, so
handle other error cases too.

BUG=285976
TEST=virtual/serviceworker/http/tests/serviceworker/registration.html

Review URL: https://codereview.chromium.org/157273005

git-svn-id: svn://svn.chromium.org/blink/trunk@166739 bbb929c8-8fbe-4397-9dbb-9b2b20218538