don't treat media file open errors other than NotFound as missing media

For example, we want to return M_UNKNOWN and propagate the error if
somebody set up their database directory permissions wrong.

src/service/media.rs

@@ -392,8 +392,12 @@ impl Service {
         key: &MediaFileKey,
     ) -> Result<Option<Vec<u8>>> {
         let path = services().globals.get_media_file(key);
-        let Ok(mut file) = File::open(path).await else {
-            return Ok(None);
+        let mut file = match File::open(path).await {
+            Ok(file) => file,
+            Err(e) if e.kind() == std::io::ErrorKind::NotFound => {
+                return Ok(None)
+            }
+            Err(e) => return Err(e.into()),
         };
 
         let mut data = Vec::new();