Add a check (and proper erroring) if your ffmpeg/GPU does not support the specified encoder

[Proryanator]

The tool doesn't handle this well today, it just tries to run for a bit, and then fails with a not so useful error.

We can run the following, ffmpeg -encoders to make sure the encoder is supported by ffmpeg as a good place to start and fail early.

In addition to this, it might be possible to have a specific error if ffmpeg does have the encoder, but your GPU does not support the given encoder (i.e. you try to run av1_nvenc on an Nvidia GPU before the 4000 series).