MultiBar::listen hangs on errors

Using the pattern in the examples on the website, MultiBar::listen will likely hang if an error occurs as the ProgressBar::finish function will never get called. It'd be nice if listen would stop blocking if there aren't any more progress bars.

Look at the MultiBar::listen implementation and the example usage pattern. Identify where errors might prevent ProgressBar::finish from being called. The fix likely involves modifying the listen loop to detect when all progress bars are finished or have errored, and then exit cleanly. Test by simulating an error in an example to reproduce the hang.

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