Recover from dead Public Gateways

[lidel]

Extracted from #639

By default Companion will redirect IPFS paths to local gateway, so even if remote HTTP server is down, content will load just fine over IPFS.

What if user does not have locally running IPFS node tho?

We are able to detect HTTP errors in WebExtensions/API/webRequest/onErrorOccurred:

Fired when a request could not be processed due to an error: for example, a lack of Internet connectivity.

The error is passed to the listener as the error property of the details object.

Note that this event is not fired for HTTP errors (4XX or 5XX responses): these will go through the normal stages of a request, calling any event listeners, and setting details.statusCode to report the error.

A rough idea would be to do what we already do for websites with DNSLink:

• detect the tab failed to load IPFS-like path
• open failed path at a public gateway in a new tab

Caveats:

• we are unable to do a HTTP redirect from onErrorOccurred so we recover in a new tab, which means this will only work for the main request (URL in location bar), won't fix images embedded from a gateway that went offline.

Prior art:

• Internet Archive's Wayback addon makes it possible to recover from HTTP 404: https://addons.mozilla.org/en-US/firefox/addon/wayback-machine_new/