Deprecated functions `DataSet.subscribe` and `DataSet.unsubscribe`

lib/DataSet.js

@@ -142,8 +142,10 @@ DataSet.prototype.on = function(event, callback) {
   });
 };
 
-// TODO: make this function deprecated (replaced with `on` since version 0.5)
-DataSet.prototype.subscribe = DataSet.prototype.on;
+// TODO: remove this deprecated function some day (replaced with `on` since version 0.5, deprecated since v4.0)
+DataSet.prototype.subscribe = function () {
+  throw new Error('DataSet.subscribe is deprecated. Use DataSet.on instead.');
+};
 
 /**
  * Unsubscribe from an event, remove an event listener
@@ -157,8 +159,10 @@ DataSet.prototype.off = function(event, callback) {
   }
 };
 
-// TODO: make this function deprecated (replaced with `on` since version 0.5)
-DataSet.prototype.unsubscribe = DataSet.prototype.off;
+// TODO: remove this deprecated function some day (replaced with `on` since version 0.5, deprecated since v4.0)
+DataSet.prototype.unsubscribe = function () {
+  throw new Error('DataSet.unsubscribe is deprecated. Use DataSet.off instead.');
+};
 
 /**
  * Trigger an event