Add a function to canonicalize URLs

We'll need this to keep track of "our" URL across 3xx redirects
Return the response status code and response headers to the caller

Surprisingly, this makes it right through to the JSON and then
to the connected client.

Errors like 404 or 500/DNS are not yet handled.
Report errors received in the "start" phase

Some errors only show up in onResourceReceived and never make it
to onResourceError.