notbrancherror blocks switch away from a deleted branch

If for some reason you have a checkout of a branch that's now deleted, you'll get a NotBranchError if you try to switch away from it, which is unfortunate:

Tue 2012-02-21 15:48:23 +1100
0.052 bazaar version: 2.6.0dev1
0.052 bzr arguments: [u'switch', u'remove-bsondump']
0.053 looking for plugins in /home/mbp/.bazaar/plugins
0.097 looking for plugins in /home/mbp/bzr/trunk/bzrlib/plugins
0.097 Plugin name launchpad already loaded
0.098 looking for plugins in /usr/lib/python2.7/dist-packages/bzrlib/plugins
0.098 Plugin name bash_completion already loaded
0.098 Plugin name changelog_merge already loaded
0.098 Plugin name launchpad already loaded
0.098 Plugin name netrc_credential_store already loaded
0.098 Plugin name news_merge already loaded
0.098 Plugin name po_merge already loaded
0.098 Plugin name weave_fmt already loaded
0.101 encoding stdout as sys.stdout encoding 'UTF-8'
0.119 Transferred: 0kB (0.0kB/s r:0kB w:0kB)
0.121 Traceback (most recent call last):
  File "/home/mbp/bzr/trunk/bzrlib/commands.py", line 930, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/commands.py", line 1141, in run_bzr
    ret = run(*run_argv)
  File "/home/mbp/bzr/trunk/bzrlib/commands.py", line 673, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/mbp/bzr/trunk/bzrlib/commands.py", line 697, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/cleanup.py", line 136, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/cleanup.py", line 166, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/home/mbp/bzr/trunk/bzrlib/builtins.py", line 6257, in run
    switch.switch(control_dir, to_branch, force, revision_id=revision)
  File "/home/mbp/bzr/trunk/bzrlib/switch.py", line 43, in switch
    _check_pending_merges(control_dir, force)
  File "/home/mbp/bzr/trunk/bzrlib/switch.py", line 69, in _check_pending_merges
    raise ex
NotBranchError: Not a branch: "/home/mbp/launchpad/lp-branches/work/.bzr/branches/936762-setuplxc-shell/".

It might be better if bzr just warned you the old branch was gone and switched without checking, or something like that.