disallow destroying relations with indices

cozo-core/src/runtime/relation.rs

@@ -584,6 +584,9 @@ impl<'a> SessionTx<'a> {
             bail!("Cannot destroy temp relation");
         }
         let store = self.get_relation(name, true)?;
+        if !stored.indices.is_empty() {
+            bail!("Cannot remove stored relation `{}` with indices attached.", name);
+        }
         if store.access_level < AccessLevel::Normal {
             bail!(InsufficientAccessLevel(
                 store.name.to_string(),