[objectives.lua] improve clarity of error message (#8780)

data/lua/wml/objectives.lua

@@ -87,7 +87,7 @@ local function generate_objectives(cfg)
 					.. color_prefix(r, g, b) .. objective_bullet.. description
 					.. turn_counter .. "</span>" .. "\n"
 			else
-				wesnoth.interface.add_chat_message "Unknown condition, ignoring."
+				wml.error("[objective] Either condition= is missing or condition= has invalid value (ignoring).")
 			end
 		end
 	end