media: ffmpeg: Add AvError::result to convert simple return codes.
media: ffmpeg: Add AvError::result to convert simple return codes.

We're going to use this pattern more in future CLs, so factor it out to
a helper.

BUG=None
TEST=cargo test -p ffmpeg &&
     cargo test --features "video-decoder,ffmpeg" -p devices video
     + v4l2r test from crosvm book

Change-Id: Ic20555f8a4ebf0403cc2f933c92bb9bb1d3f51f9
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3822432
Reviewed-by: Alexandre Courbot <301cd6a3ea50a894b4b51e385c5eb12b9a4ffc0c@chromium.org>
Commit-Queue: Tatsuyuki Ishi <9fb0affb047566c1ca29d70049b0f579b84f88dc@google.com>
Tested-by: Tatsuyuki Ishi <9fb0affb047566c1ca29d70049b0f579b84f88dc@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm