Retries on Service Unavailable
Sometimes servers or proxy may return a 503 Service Unavailable or 504 Gateway Timeout.
When it happens on a JSON-RPC request, the current actions (usually the tab) can no more be trusted (and thus be used). But as such error may be temporary, we should retry the request a few times before failing. The headers may contain a Retry-After
value which define the delay to wait but I think we should try earlier if the value is too long for the user to wait.