Worker-side Checks about proxima

[in03]

This might require some thought actually...
We'd have to make a decision about putting these worker side checks either in the worker launcher or per worker initialization. In the latter case, that means every single worker is going to complain if they're all running on the same host. The worker launcher is still very windows-centric atm. Linux and Mac users can properly daemonize and prefork and do whatever they want, so they don't really need the launcher. That's probably a project / issue in itself. If we did the checks in the worker launcher, Linux and Mac users would likely miss out.

[in03]

Before receiving any tasks (which are darn hard to debug with all the Celery fluff and bubble), let's run a bunch of quick permissions / access tests. E.g:

• Can we access the proxy_media path? Do we have R/W permissions?
• Are there any configuration conflicts?

Then we can pop up some user-friendly warnings before attempting any encoding.

Proxy media path is actually set by the queuer and defined in the job task I believe, so there's no easy way to do this except at runtime.