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

[zambrovski]

Steps to reproduce

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

Copied from holunda-io/camunda-bpm-taskpool#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...