rename Error::Image to Error::GenerateThumbnail

The previous error was nonspecific and, because of the '#[from]'
annotation, could have been used accidentally for *any* errors from the
image crate.

src/service/media.rs

@@ -276,10 +276,12 @@ impl Service {
 
         t::debug!("Serializing thumbnail as PNG");
         let mut thumbnail_bytes = Vec::new();
-        thumbnail.write_to(
+        thumbnail
+            .write_to(
                 &mut Cursor::new(&mut thumbnail_bytes),
                 image::ImageFormat::Png,
-        )?;
+            )
+            .map_err(Error::GenerateThumbnail)?;
 
         Ok(Some(thumbnail_bytes))
     }

src/utils/error.rs

@@ -36,11 +36,8 @@ pub(crate) enum Error {
         #[from]
         source: rocksdb::Error,
     },
-    #[error("Could not generate an image.")]
+    #[error("Could not generate image thumbnail.")]
-    Image {
+    GenerateThumbnail(#[source] image::error::ImageError),
-        #[from]
-        source: image::error::ImageError,
-    },
     #[error("Could not connect to server: {source}")]
     Reqwest {
         #[from]