Clean up the kappa path detection code a bit (#1)
Clean up the kappa path detection code a bit (#1)

* Avoid shadowing builtin 'dir'
* Remove comment text about checking for directory-ness
* Clean up logical flow in a couple of places
* Add more information to some exception messages
jmuhlich/pysb,pysb/pysb,alubbock/pysb,johnbachman/pysb,johnbachman/pysb,LoLab-VU/pysb,LoLab-VU/pysb