Connection errors

* Better error message if the request to the Marathon API fails.
  It now displays an understandable sentence, and not just a network error code.
* Reload the config after a zookeeper reconnection.

See #12
Is error.message the right thing to extract? You could just decide to depend on the toString logic of the Error. But I've also seen error.code being used. You may be also be interested in a stack (although your own error may give the same, but richer stack).