fixes elm#952

Checking callback type on port subscribe

Currently calling `elm.ports[portName].subscribe(undefined)` will cause an exception only when subscribers get called, this commit adds a type check before pushing the callback on subscription list

src/Elm/Kernel/Platform.js

@@ -389,6 +389,11 @@ function _Platform_setupOutgoingPort(name)
 
 	function subscribe(callback)
 	{
+		if (typeof callback !== 'function')
+		{
+			throw new Error('Trying to subscribe an invalid callback on port `' + name + '`');
+		}
+
 		subs.push(callback);
 	}