txn_limbo: panic when synchro WAL write fails

CONFIRM and ROLLBACK go to WAL. Their WAL write can fail just like
any other WAL write. However it is not clear what to do in that
case, especially in case of ROLLBACK fail.

The patch adds panic() stub so as to at least terminate the
instance. Before the patch it would work like nothing happened,
with undefined behaviour.

Closes #5159