Ugly bug marking, but test suite passing for python2.6. Since 2.6
is still developed investigating now seems a waste.

* bzrlib/tests/tree_implementations/test_walkdirs.py:
(TestWalkdirs.test_walkdir_root,
TestWalkdirs.test_walkdir_subtree): Mark failing tests.

* bzrlib/tests/tree_implementations/test_test_trees.py:
(TestTreeShapes.test_tree_with_subdirs_and_all_content_types):
Mark failing tests.

* bzrlib/tests/tree_implementations/test_inv.py:
(TestEntryDiffing.test_link_diff_changed,
TestInventory.test_symlink_target, TestInventory.test_symlink):
Mark failing tests.

* bzrlib/errors.py:
(BzrError.__str__, BzrError.__str__): Working around python2.5
hiding'message' attribute is not needed anymore.