Add comment explaining placement of system headers in C++ Python module
Raise Python exceptions if CaffeNet input files don't exist

This is for convenience only; if the input files are moved or deleted at
the wrong time, pycaffe could still crash (as it did before). However,
in most cases this will make interactive use easier.
Merge pull request #228 from longjon/pycaffe-exceptions

Raise Python exception if input files don't exist for better interactivity