Improve snapshot filter warning message

Include the root-cause why the snapshot prefix is ignored.

cmd/restic/find.go

@@ -31,7 +31,7 @@ func FindFilteredSnapshots(ctx context.Context, repo *repository.Repository, hos
 				} else {
 					id, err = restic.FindSnapshot(ctx, repo, s)
 					if err != nil {
-						Warnf("Ignoring %q, it is not a snapshot id\n", s)
+						Warnf("Ignoring %q: %v\n", s, err)
 						continue
 					}
 				}