Handle ajax errors

[wvengen]

Currently errors during an ajax request are not handled, the user sees nothing. This may happen, for example, with the delta_input buttons in the order and balancing screen to change group_order_article amounts.

[wvengen]

This is, unfortunately, not standard in Rails.

There are two cases here:

1. An error condition is found in the code, or
2. a server error occurs (resulting in an http 500 status code).

The second can be caught by jQuery's ajaxError() handler, and the first one too - when we return an http status code as suggested elsewhere too.

For validation errors, 422 would be suitable; what to use for other unexpected conditions? (E.g., when no member can be found for an ordergroup in foodcoop-adam#106.)

[wvengen]

The commit also removes non-working ajax hooks; if you we, at some point, want to disable the submit button after submitting remote forms, this would be possible: http://www.railsonmaui.com/blog/2014/02/23/simple-form-and-disable-processing-by-default/
(even though I couldn't get it to work now).