[merge][#330248] don't crash on incorrect progress bar updates

[Martin Pool]

The UI model is that there is one correctly-nested progress bar stack
at any time; this should correspond to a nested set of try/finally
blocks that close them off as tasks complete.  There is code that
tries to check it and issue warnings, but if you try to finish a
progress bar when none are active at all it crashes bzr, as seen in
<https://bugs.edge.launchpad.net/bzr/+bug/330248>.

In that bug the root cause is probably
<https://bugs.edge.launchpad.net/bzr/+bug/303568> but 330248 is for
the knock-on mess in the progress bar.

Arguably we should allow an arbitrary forest of progress tasks, and
arguably we should not show any warnings if the program gets it wrong,
though there have been a couple of cases where it found bugs.  I
intend to send a patch that at least suppresses them for release
builds.  But I think this change is still worthwhile as it is.

-- 
Martin <http://launchpad.net/~mbp/>
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