breaks when unable to create cache directory

[Jelmer Vernooij]

This is a regress from the new bedding code:

Traceback (most recent call last):
  File "tools/generate_docs.py", line 110, in <module>
    main(sys.argv)
  File "tools/generate_docs.py", line 79, in main
    with breezy.initialize():
  File "breezy/__init__.py", line 222, in initialize
    state._start()
  File "breezy/library_state.py", line 98, in _start
    self._trace.__enter__()
  File "breezy/trace.py", line 655, in __enter__
    self._original_state = enable_default_logging()
  File "breezy/trace.py", line 292, in enable_default_logging
    brz_log_file = _open_brz_log()
  File "breezy/trace.py", line 249, in _open_brz_log
    _brz_log_filename = _get_brz_log_filename()
  File "breezy/trace.py", line 210, in _get_brz_log_filename
    return os.path.join(bedding.cache_dir(), 'brz.log')
  File "breezy/bedding.py", line 187, in cache_dir
    os.makedirs(cache_dir)
  File "/usr/lib/python2.7/os.py", line 150, in makedirs
    makedirs(head, mode)
  File "/usr/lib/python2.7/os.py", line 150, in makedirs
    makedirs(head, mode)
  File "/usr/lib/python2.7/os.py", line 157, in makedirs
    mkdir(name, mode)
OSError: [Errno 13] Permission denied: '/sbuild-nonexistent'

[Jelmer Vernooij (jelmer)]

Note that this only affects 3.1.