Fix silent FFMPEG failure

[RobRoyce]

This pull request addresses issue #26 by ensuring an error message is displayed when the trimesh, pyrender, and/or JITRenderer modules fail to import due to a missing ffmpeg library, rather than allowing the process to fail silently.

Note that this use of try/except is still problematic and should probably be addressed in future updates. But at least this temporary fix will keep people from spending a lot of time trying to figure out the issue.

[VincentCCandela]

SOLUTION:

1. install ffmpeg on your system
2. put this in your main runtime to make sure that your code points correctly to your ffmpeg binary

import os
os.environ["IMAGEIO_FFMPEG_EXE"] = "/usr/bin/ffmpeg" (replace with the correct ffmpeg path)