avisynth: Cosmetics
avisynth: Exit gracefully when trying to serve video from v2.5.8.

'Fixes' ticket #2526 insofar as it stops 2.5.8 from crashing and
tells the user to upgrade to 2.6 if they want to make video input
work. A real solution to #2526 would be to get video input from
2.5.8 to work right.