Improve debug-log output when charms have uncaught exceptions

[tonyandrewmeyer]

@sed-i suggested that having the charm revision and possibly other information would be beneficial when end-users are doing bug reporting.

It feels like the revision is perhaps too specific (and complicated since we don't have a good way to get a canonical "revision" inside the unit) but maybe we can make it easier for the charm to provide additional information to include in the log at this point, or perhaps there should be a (jhack?) tool that makes it easier to collect quality bug report information with assistance from ops.

@dimaqq also suggested (this might be a separate feature) that we might want to catch 'uncaught' exceptions and re-raise them to provide a cleaner traceback (an issue that @PietroPasotti raised re: tracing wrappers) or otherwise present them.