User task form for should react on error if the task can not be loaded

### Steps to reproduce

* camunda-bpm-taskpool version: any
* Camunda BPM version: any
* JDK version: any
* Operating system: any
* Steps:

Copied from https://github.com/holunda-io/camunda-bpm-taskpool/issues/273

If I change permission and open a task I'm not allowed to, the form is loaded (empty) instead of informing that the task can't be loaded.

### Expected behaviour

A task is not shown if it is not allowed to be edited. If the task is accessed somehow else, the erro should appear instead of an empty form.

### Actual behaviour

Empty form is shown.

I observed (with help of in-memory single node scenario), that the frontend is lacking refresh requests after a task is completed. This is the reason why non-existent tasks appear in the list and why wrong requests are sent out from the frontend...