[merge] show locale, traceback in plugins

[Martin Pool]

> how about:
>     err_file.write("plugins:\n")
>     for a_plugin in plugin.plugins():
>         err_file.write("  %s [%s]" %
>             (a_plugin.path(), a_plugin.__version__))

I think that needs to be a_plugin.get since they may not declare a
version.  I would rather keep it a bit more compact and have them on
one line people can get the path from bzr plugins, and that's not
likely to be particularly interesting to people seeing the bug report
-- or is it?

-- 
Martin