Ignore path if we're not allowed to access it.

Don't try to list the contents of a path if we don't have access to it.

This issue is particularly common on Windows where user directories are more likely to contain protected paths. Running the [tree editor traitsui demo](https://github.com/enthought/traitsui/blob/master/examples/demo/Advanced/Adapted_tree_editor_demo.py) from the user's root directory in Windows will reproduce this error (at least on my system, the `Application Data` directory gives errors). Alternatively, you can run the example with a dummy directory:

    mkdir nonreadable
    chmod a-r nonreadable
    python Adapted_tree_editor_demo.py