OSError from _readdir is printed without a useful filename

When an OSError occurs, eg because of an environmental problem, bzr tends to print it without the filename, which makes it harder to debug.

The python error has a filename attribute; we should print it.

Example:

No handlers could be found for logger "bzr"
bzr: ERROR: exceptions.OSError: [Errno 13] Permission denied

Traceback (most recent call last):
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 896, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 842, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 542, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 856, in ignore_pipe
    result = func(*args, **kwargs)
  File "/usr/lib/python2.5/site-packages/bzrlib/builtins.py", line 241, in run
    show_pending=(not no_pending), verbose=verbose)
  File "/usr/lib/python2.5/site-packages/bzrlib/status.py", line 118, in show_tree_status
    want_unversioned=want_unversioned)
  File "/usr/lib/python2.5/site-packages/bzrlib/tree.py", line 95, in changes_from
    want_unversioned=want_unversioned,
  File "/usr/lib/python2.5/site-packages/bzrlib/decorators.py", line 138, in read_locked
    result = unbound(self, *args, **kwargs)
  File "/usr/lib/python2.5/site-packages/bzrlib/tree.py", line 877, in compare
    want_unversioned=want_unversioned)
  File "/usr/lib/python2.5/site-packages/bzrlib/delta.py", line 230, in _compare_trees
    want_unversioned=want_unversioned):
  File "_dirstate_helpers_c.pyx", line 1347, in _dirstate_helpers_c.ProcessEntryC.__next__
  File "_dirstate_helpers_c.pyx", line 1613, in _dirstate_helpers_c.ProcessEntryC._iter_next
  File "_dirstate_helpers_c.pyx", line 1772, in _dirstate_helpers_c.ProcessEntryC._loop_one_block
  File "/usr/lib/python2.5/site-packages/bzrlib/osutils.py", line 1401, in _walkdirs_utf8
    dirblock = sorted(read_dir(relroot, top))
  File "_readdir_pyx.pyx", line 237, in _readdir_pyx.UTF8DirReader.read_dir
  File "_readdir_pyx.pyx", line 303, in _readdir_pyx._read_dir
OSError: [Errno 13] Permission denied