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
Do you suggest something like this:
```javascript
function reloadConfig() {
  clearTimeout(reloadTimeout);
  reloadTimeout = null;

  marathon.getTasks(function (err, tasks) {
    if (err && !reloadTimeout) {
       reloadTimeout = setTimeout(reloadConfig, config.marathon.retryDelay);
       console.error(err);
       return;
    }
    generator.render({ tasks: tasks }, function (err) {
      if (err) {
        console.error(err);
        return;
      }
      proxy.reload(function (err) {
        if (err) {
          console.error(err);
          return;
        }
      });
    });
  });
}
```