refactor(tvix/castore/blobservice): use io::Result in trait

For all these calls, the caller has enough context about what it did, so
it should be fine to use io::Result here.

We pretty much only constructed crate::Error::StorageError before
anyways, so this conveys *more* information.

Change-Id: I5cabb3769c9c2314bab926d34dda748fda9d3ccc
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10328
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>

tvix/castore/src/import.rs

@@ -117,7 +117,10 @@ async fn process_entry<'a>(
             return Err(Error::UnableToRead(entry.path().to_path_buf(), e));
         };
 
-        let digest = writer.close().await?;
+        let digest = writer
+            .close()
+            .await
+            .map_err(|e| Error::UnableToRead(entry.path().to_path_buf(), e))?;
 
         return Ok(Node::File(FileNode {
             name: entry.file_name().as_bytes().to_vec().into(),