OSError in ensure_config_dir_exists shouldn't be fatal

If you run bzr 2.0.4 as a user whose home directory doesn't exist you get-

escudero% bzr -Derror clean-tree
failed to open trace file: [Errno 2] No such file or directory: '/home/bzr-web/.bzr.log'
bzr: ERROR: exceptions.OSError: [Errno 2] No such file or directory: '/home/bzr-web/.bazaar'

Traceback (most recent call last):
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 842, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 1037, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 654, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.5/site-packages/bzrlib/builtins.py", line 5746, in run
    dry_run=dry_run, no_prompt=force)
  File "/usr/lib/python2.5/site-packages/bzrlib/clean_tree.py", line 55, in clean_tree
    ignored=ignored, detritus=detritus))
  File "/usr/lib/python2.5/site-packages/bzrlib/clean_tree.py", line 40, in iter_deletables
    if tree.is_ignored(subp):
  File "/usr/lib/python2.5/site-packages/bzrlib/workingtree.py", line 1748, in is_ignored
    self._ignoreglobster = globbing.Globster(self.get_ignore_list())
  File "/usr/lib/python2.5/site-packages/bzrlib/workingtree.py", line 1723, in get_ignore_list
    ignore_globs.update(ignores.get_user_ignores())
  File "/usr/lib/python2.5/site-packages/bzrlib/ignores.py", line 129, in get_user_ignores
    _set_user_ignores(USER_DEFAULTS)
  File "/usr/lib/python2.5/site-packages/bzrlib/ignores.py", line 151, in _set_user_ignores
    config.ensure_config_dir_exists()
  File "/usr/lib/python2.5/site-packages/bzrlib/config.py", line 772, in ensure_config_dir_exists
    os.mkdir(path)
OSError: [Errno 2] No such file or directory: '/home/bzr-web/.bazaar'

we should cope without it.