refactor(tvix/castore/import): restructure error types

Have ingest_entries return an Error type with only three kinds:

 - Error while uploading a specific Directory
 - Error while finalizing the directory upload
 - Error from the producer

Move all ingestion method-specific errors to the individual
implementations.

Change-Id: I2a015cb7ebc96d084cbe2b809f40d1b53a15daf3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11557
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Connor Brewster <cbrewster@hey.com>

tvix/store/src/import.rs

@@ -117,7 +117,9 @@ where
     DS: AsRef<dyn DirectoryService>,
     PS: AsRef<dyn PathInfoService>,
 {
-    let root_node = ingest_path(blob_service, directory_service, path.as_ref()).await?;
+    let root_node = ingest_path(blob_service, directory_service, path.as_ref())
+        .await
+        .map_err(|e| std::io::Error::new(std::io::ErrorKind::Other, e))?;
 
     // Ask the PathInfoService for the NAR size and sha256
     let (nar_size, nar_sha256) = path_info_service.as_ref().calculate_nar(&root_node).await?;