improved UI while simplifying AJAX form handling

moving form validation errors into the respective form puts the
respective messages in proper context

the slight increase in server-side complexity allows for simpler APIs
and less awareness on the client side

note, however, that pjax operations do not update global messages, which
might be a significant issue in a real application

this reverts the previous commit
(6873b3fee5fd565799a9ab85b951a3d05219aa00)