Optional path checks

[markfinger]

The path checks in the python side are not overly useful as the node process will detect the same issue when it tries to import the file. That being said, the python errors are more explicit and will be more familiar for python folk.

Primary benefit of this would be a saner setup for docker folk who have some eccentric mount paths. See #75

Recommend adding an optional setting that removes the os.exists checks in

python-react/react/render.py

Line 14 in 1bad117

if not os.path.exists(path):

[markfinger]

I don't have any need for this myself, so I'm happy to wait out a possible PR.

If someone would like to take this on, you'll need to tweak the aforementioned line in render.py, and you'll need to add another setting in react/conf.py