Undo fails if any of the following is true:
- cell_list is empty (should never happen, add an assertion for this)
- all connections get a ConnectionClosed exception
- transaction cannot be found in any storage
- an unexpected answer was received (it did not change txn_info)

This fixes the first case, as the test might have received old values from
former executions (which could be neither -1 nor 0) in the same
transaction.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@1832 71dcc9de-d417-0410-9af5-da40c76e7ee4