Tasks and Questions with explicit pre-/post-conditions

[mkutz]

Some Tasks and Questions aren't top-level. They build upon other Tasks have been performed before.

This is not really a technical problem, after all pre-conditions can be checked first thing in such performAs methods. However, this is kind of implicit and sometimes hard to recognize at a glance at the code.

It is also sometimes interesting to check post-conditions after a Task to reduce pre-checking and to fail faster and in the right context. For example after clicking the "register" button wait for either a page transition or an error message.

The suggestion would be do create two new interfaces Preconditioned and Postconditioned, which require the implementation of checkPrecondition/checkPostcondition which will automatically called by the Actor in does and checks.

Retryables need to be considered here! To be consistent with current behaviour I think it would be best to perform the check at every retry.