Revert patch lp:1121297 since it breaks error reporting.
reset_counters() was called before error reporting.
Assure TaskQueue has the right defaults when starting and finishing.
Change plugin installation.
 - make sure pipeline restarts after installation.
 - wait while installation is pending.
 - if user cancel installation, don't even start the following ones.
Skip renaming if errors.
vfs_rename returns an error.
New emit_async() so 'finished' signals will not be run before the
real end of the task.