Make tingbot-python stricter with case sensitivity of paths on Mac

[joerick]

A classic bug in running apps on Tingbot is that the case of a path to an image or font is different from the file system to the program.

Such bugs don't occur in the simulator on Mac, because HFS+ (ding) is case insensitive. To minimise bugs that expose themselves on the Pi, we should make these error on Mac as well, by adding an explicit check for the file before accessing it.