Pass flow failures to task's revert method

Now when tasks are reverted, all flows failures seen so far are passed
to revert method as 'flow_failures' keyword argument, as a dictionary
task name => misc.Failure.

Closes-bug: #1246616
Change-Id: I81f8c10684d54aaf5c31bb3badb6afe4c949f680
Merge "Pass flow failures to task's revert method"