Response (HTTP) status codes for protocol handlers

[Gozala]

This is followup for #2 where you can find all details and context. Below is a quote summarizing it

Response (HTTP) status codes - Turns out nsIHttpChannel reports that by passing nsresult as statusCode parameter to onStopRequest of nsIStreamListener which was passed to channel's asyncOpen. I've asked necko team to point me to the source that does mapping between HTTP status codes and nsresult codes and once I have that it should be fairly easy to support status codes. Please note that very likely things like redirects caching and alike won't just magically start doing right things, most likely those don't have corresponding nsresult it's just implementation of protocol handler does the right thing there instead, which is to say doing caching is on the protocol implementer. Not sure about redirects I'm inclining to just let protocol handler return response with different url and data stream than requested as it would reduce messaging overhead but we'll see after there's more clearance on 1. Async Redirects.

[Gozala]

Migrate to https://bugzilla.mozilla.org/show_bug.cgi?id=1572215