Improve reporting of the final stats in case of error

[abyss7]

Right now we make attempt to collect the final stats from all CAs into KQP Executer. But by many reason we may not collect the full stats, i.e. node may become unavailable, CA may hang and report its stats at all, etc.

To indicate inconsistencies the KQP Executer should always report stats about all tasks - and in case of missing stats specify the reason.

This way the client may decide how to use the final stats, i.e. merge them with previous periodic stats or just drop incomplete stats completely.