Add core.spawn_tree_on_vmanip (#15415)

This function works like `core.spawn_tree`, but spawns an L-system tree onto a VoxelManip object instead on the map.

src/mapgen/treegen.cpp

@@ -875,4 +875,15 @@ void make_pine_tree(MMVManip &vmanip, v3s16 p0, const NodeDefManager *ndef,
 	}
 }
 
+std::string error_to_string(error e)
+{
+	switch (e) {
+		case SUCCESS:
+			return "success";
+		case UNBALANCED_BRACKETS:
+			return "closing ']' has no matching opening bracket";
+	}
+	return "unknown error";
+}
+
 }; // namespace treegen