Fix blurhash computation for some animations

In the pure-rust blurhash path, I recently added byte streaming for the
image reader, which for some formats (like gif) can produce and operate
on the first frame of an animation without downloading the whole file.

This means that a blurhash could be calculated and the reader dropped
before the writer had written all the bytes to the channel. Erroring in
this case is incorrect.

src/blurhash.rs

@@ -97,7 +97,11 @@ where
             let mut streamer = stream.into_streamer();
 
             while let Some(res) = streamer.next().await {
-                tx.send(res).await.map_err(|_| UploadError::Canceled)?;
+                if tx.send(res).await.is_err() {
+                    // not sending isn't an error, we can sometimes compute blurhashes without the
+                    // entire body
+                    break;
+                }
             }
             drop(tx);