virtio: video: Add an Error implementation for TryFromFormatError.

As a convention, error types should always implement Error, so we're
doing it here.

The traits are manually implemented as opposed to using ThisError; it
was simple enough, and using ThisError requires making the error type an
enum which makes it more verbose to construct.

BUG=None
TEST=cargo build --features "video-decoder,ffmpeg"

Change-Id: If94926c1b2f14295b29c877c2c70d361b4adee90
Reviewed-on: https://chromium-review.googlesource.com/c/crosvm/crosvm/+/3925332
Commit-Queue: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>

devices/src/virtio/video/ffmpeg.rs

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 use std::error::Error;
+use std::fmt::{Display, Formatter};
 
 use thiserror::Error as ThisError;
 
@@ -131,6 +132,17 @@ impl TryAsAvFrameExt for GuestResource {
 #[derive(Debug)]
 pub struct TryFromFormatError(());
 
+impl Display for TryFromFormatError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        write!(
+            f,
+            "No matching format to convert between AvPixelFormat and Format"
+        )
+    }
+}
+
+impl Error for TryFromFormatError {}
+
 impl TryFrom<Format> for AvPixelFormat {
     type Error = TryFromFormatError;