check: Present a human-readable error when gst-python is not working properly

This is 1000x better than the cryptic traceback users get when pygst is missing
check: Move dependencies to a single location and document for packagers
PEP8 compliance fixes in settings.py and render.py
check: Clean up the soft dependencies constants mess

This file is meant to make the packagers' job easier,
so it must be as legible as possible.