Better error handling for PatientDatabase constructor

[scottfleming]

Is your feature request related to a problem? Please describe.
Error messages for loading a PatientDatabase object with an invalid argument are cryptic and unhelpful

Describe the solution you'd like
Better error handling and a more clear description of the problem e.g., "Path passed to PatientDatabase constructor was invalid: {invalid path}"

[EthanSteinberg]

piton.datasets.PatientDatabase(None) now returns a better error message. The non existent directory error still needs to get improved.