adjust to new error formats

[gr2m]

Okay, that might sound confusing. What I mean is that we want to normalize the frontend error handling by following JavaScript's Error conventions of having a name and a message property. At the moment, we usually (but not always) followed CouchDB's way having an error and a reason property.

As we change it throughout the code in the frontend, it will be needed to be changed in the backend as well, at least where objects are updated with an $error property, like in
https://github.com/hoodiehq/hoodie-plugin-users/blob/master/lib/password_reset.js#L53

That should now read

$error: {
  name: 'HoodieNotFoundError',
  message: util.format('user %s could not be found', username),
  username: username
}

If you don't see any reasons, I'd let you know as soon as the frontend code code is ready, so we can publish all at once:
hoodiehq/hoodie#173

Hope that makes all sense?