ignore missing or bad previous index

This is not a fatal condition, it just means that we cannot do
incremental backups client-side. This can happen if a new disk was added
the a VM since the last backup for example.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>

src/commands.rs

@@ -114,7 +114,11 @@ pub(crate) async fn register_image(
 
     let index = match manifest {
         Some(manifest) => {
-            Some(client.download_previous_fixed_index(&archive_name, &manifest, known_chunks.clone()).await?)
+            match client.download_previous_fixed_index(&archive_name, &manifest, known_chunks.clone()).await {
+                Ok(index) => Some(index),
+                // not having a previous index is not fatal, so ignore errors
+                Err(_) => None
+            }
         },
         None => None
     };