clarify/fix error reporting on ajax calls

[testbird]

An issue to track the status of ajax error reporting, for example when form submission fails.

The "*_form" template is rendered on error and was used to display errors in
the form element.
Currently there are calls to *_form.erb files, but they are not found.

[testbird]

Curiously, form related flashmessages showed up when switching from properties page to classes page in the admin interface right after a form submission failure.