Warn instead of completely removing minetest.digprop_whatever() Lua functions

share/server/builtin.lua

@@ -749,6 +749,21 @@ minetest.register_item(":", {
 	type = "none",
 })
 
+--
+-- Default material types
+--
+function digprop_err()
+	error("The minetest.digprop_* functions are obsolete and need to be replaced by item groups.")
+end
+
+minetest.digprop_constanttime = digprop_err
+minetest.digprop_stonelike = digprop_err
+minetest.digprop_dirtlike = digprop_err
+minetest.digprop_gravellike = digprop_err
+minetest.digprop_woodlike = digprop_err
+minetest.digprop_leaveslike = digprop_err
+minetest.digprop_glasslike = digprop_err
+
 --
 -- Creative inventory
 --