Improve error when symlink cannot be accessed during walk

[pivotal-david-osullivan]

Summary

The file walk used by libbs e.g. to create a listing of the source/workspace directory can fail if it encounters a directory symlink that it cannot follow - this PR improves the error relating to this situation

Use Cases

User source path contains a symlink dir/file that cannot be followed

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[dmikusa]

Thanks! I'm pretty sure I've seen that come up. This will make it a lot clearer.