Don't emit ERROR until a keepalive poke fails
This accomplishes the same as
https://github.com/matrix-org/matrix-js-sdk/pull/216/files, but
without the client waiting 110 seconds for a sync request to time
out. That is, don't display an error message a soon as a sync
request fails, since we should accept that sometimes long lived
HTTP connections will go away and that's fine.
Also:
* Use request rather than deprecated requestWithPrefix
* http-api: The empty string may be falsy, but it's a valid prefix
