VapourSynth: missing calls to VSAPI::setError

[dubhater]

vapoursynth/vapoursynth#226

It looks like you return from vs_lwlibavsource_create without calling createFilter and without calling setError. Please always call setError when there is an error. I was going to list all the places where you're not calling it, but there are over 9000 layers and places where you may or may not have called it, and I can't tell anymore.

Bonus: these are not necessary:

• L-SMASH-Works/VapourSynth/lwlibav_source.c

  Line 270 in 9828349

  if( !file_path )

• L-SMASH-Works/VapourSynth/libavsmash_source.c

  Line 319 in 9828349

  if( !file_name )

propGetData will not fail because the "source" parameter is not optional. Its presence is checked before vs_lwlibavsource_create or vs_libavsmashsource_create are called.