[DataAvailability] add a check for executors passed by a client

[illia-malachyn]

Clients can pass the required executors' IDs and required executor number via the criteria. We need to add verify that those executors indeed exist and that their number is not greater than the real number of executors.

As a part of this task, or subtask, or a related task, we need to handle a case when

1. the users states he wants ALL executors produce the result
2. one of the executors crashes and cannot produce the result for some time (e.g. it restarts and unavailable for 5-10 minutes)
3. we need to have a way to tell a user that his criteria CANNOT be met currently.

During the discussion, we decided to add such a check: "if a block is already sealed but the criteria is still not met, return an error to the user saying that the criteria cannot be met."