recipes that use "." for a nest command do not fail gracefully

What recipe are you trying to build?
Do you have the same problem if you run the recipe with bzr-builder locally?

An example recipe is this:
# bzr-builder format 0.2 deb-version 0+01234
lp:dftd-daily
nest packaging lp:~aegirxx-googlemail/dftd-daily/packaging-daily-src .

The lp:~aegirxx-googlemail/dftd-daily/packaging-daily-src contains the debian directory, when instead it should be the contents of the debian directory, and be nested like this:
nest packaging lp:~aegirxx-googlemail/dftd-daily/packaging-daily-src debian

However, bzr-builder should reject the recipe as specifying an invalid directory, rather than failing with a lock error.

Example failure:
$ bzr -Derror dailydeb --no-build recipe
Exception AttributeError: "'NoneType' object has no attribute 'close'" in <bound method SmartSSHClientMedium.__del__ of SmartSSHClientMedium(bzr+ssh://<email address hidden>/)> ignored
Exception AttributeError: "'NoneType' object has no attribute 'close'" in <function terminate at 0x3960050> ignored
Exception AttributeError: "'NoneType' object has no attribute 'close'" in <bound method SmartSSHClientMedium.__del__ of SmartSSHClientMedium(bzr+ssh://<email address hidden>/)> ignored
Exception AttributeError: "'NoneType' object has no attribute 'close'" in <function terminate at 0x3976398> ignored
Exception AttributeError: "'NoneType' object has no attribute 'close'" in <bound method SmartSSHClientMedium.__del__ of SmartSSHClientMedium(bzr+ssh://<email address hidden>/)> ignored
Exception AttributeError: "'NoneType' object has no attribute 'close'" in <function terminate at 0x3af87d0> ignored
Unable to obtain lock file:///tmp/bzr-builder-2O0HnA/recipe-0%2B01234/ held by Aaron Bentley <email address hidden>
at lumpy [process #1638], acquired 0 seconds ago.
Will continue to try until 12:54:00, unless you press Ctrl-C.
See "bzr help break-lock" for more.
bzr: ERROR: bzrlib.errors.LockContention: Could not acquire lock "(local)": file:///tmp/bzr-builder-2O0HnA/recipe-0%2B01234/

Traceback (most recent call last):
  File "/home/abentley/bzr/bzr.dev/bzrlib/commands.py", line 911, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/home/abentley/bzr/bzr.dev/bzrlib/commands.py", line 1111, in run_bzr
    ret = run(*run_argv)
  File "/home/abentley/bzr/bzr.dev/bzrlib/commands.py", line 689, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/home/abentley/bzr/bzr.dev/bzrlib/commands.py", line 704, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/home/abentley/bzr/bzr.dev/bzrlib/cleanup.py", line 135, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/home/abentley/bzr/bzr.dev/bzrlib/cleanup.py", line 165, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/home/abentley/.bazaar/plugins/builder/__init__.py", line 527, in run
    build_tree(base_branch, working_directory)
  File "/home/abentley/.bazaar/plugins/builder/recipe.py", line 340, in build_tree
    instruction.apply(target_path, tree_to, br_to)
  File "/home/abentley/.bazaar/plugins/builder/recipe.py", line 391, in apply
    target_path=os.path.join(target_path, self.nest_path))
  File "/home/abentley/.bazaar/plugins/builder/recipe.py", line 332, in build_tree
    tree_to.lock_write()
  File "/home/abentley/bzr/bzr.dev/bzrlib/workingtree_4.py", line 629, in lock_write
    self.branch.lock_write()
  File "/home/abentley/bzr/bzr.dev/bzrlib/branch.py", line 2425, in lock_write
    self.control_files.lock_write(token=token))
  File "/home/abentley/bzr/bzr.dev/bzrlib/lockable_files.py", line 187, in lock_write
    token_from_lock = self._lock.lock_write(token=token)
  File "/home/abentley/bzr/bzr.dev/bzrlib/lockdir.py", line 606, in lock_write
    return self.wait_lock()
  File "/home/abentley/bzr/bzr.dev/bzrlib/lockdir.py", line ...