datastore: add additional context for get_owner parsing error

By this it becomes clear that the error stems from a parsing error when
getting the backup group owner.

See also: https://forum.proxmox.com/threads/139482/

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>

pbs-datastore/src/datastore.rs

@@ -602,7 +602,12 @@ impl DataStore {
     ) -> Result<Authid, Error> {
         let full_path = self.owner_path(ns, backup_group);
         let owner = proxmox_sys::fs::file_read_firstline(full_path)?;
-        owner.trim_end().parse() // remove trailing newline
+        owner
+            .trim_end() // remove trailing newline
+            .parse()
+            .map_err(|err| {
+                format_err!("parsing owner for {backup_group} failed: {err}")
+            })
     }
 
     pub fn owns_backup(