Cleanup error handling and enums

Cargo.lock

Cargo.toml

lib/ffmpeg_api/Cargo.toml

lib/ffmpeg_api/src/api.rs

lib/ffmpeg_api/src/enums.rs

lib/ffmpeg_api/src/err.rs

lib/ffmpeg_api/src/err_av.rs

lib/ffmpeg_api/src/err_ffi.rs

lib/ffmpeg_api/src/lib.rs

lib/media_time/Cargo.toml

lib/media_time/src/errors.rs

lib/media_time/src/lib.rs

lib/media_time/src/media_time.rs

lib/webvtt/Cargo.toml

lib/webvtt/src/webvtt.rs

src/lib.rs

src/main.rs

src/spritesheet.rs