Handle connection errors more gracefully

I've worked out that we can go to the Advanced tab and set, under Custom Options, something like timeout: 10. This works as intended, albeit that if that timeout is ever exceeded, it throws a fatal error that Webform module can't recover from. It looks like Guzzle is throwing a ConnectException on timeout, which isn't classed as a RequestException. This is also a problem with Webform's own RemotePostWebformHandler, but I don't see a reason why we shouldn't fix this here.