Report string error paths as a JSON Pointer

Whenever error paths are strings, the default, report them as a JSON Pointer.

Fixes #59 
I'm the same, especially when mucking with someone else's code base.  I would rather be explicit and easy to read.  Thanks again for accepting this, with these last two PRs I can move my jjv+jjve code base to z-schema.  If I can help further, let me know.