Add docs for request error handling

[Fneufneu]

Examples miss a really important point: error handly for a request like a simple timeout.
simply like this:

$request = $client->request('GET', $url);
$request->on('error', function ($e) {
  $socketerror = $e->getPrevious();
  echo "request failed with error: " . $e->getMessage() . PHP_EOL;
});

I spend too much time just for this so i think it should be added in the README or the exemple section.

[WyriHaximus]

@Fneufneu sounds reasonable, could you create a PR adding documentation for this?

[WyriHaximus]

@Fneufneu So @clue already fixed it though: #97