Check if passed object is task earlier

[ondrejsevcik]

Hi, it happened to me a few times that I had a typo in the task name that I've passed in into perform helper and then I got an error when I clicked the button.

The first argument passed to the perform helper should be a Task object (without quotes); you passed undefined, so I don't have to find it later when I click the button. 

But it would be nicer to get the error immediately when the template is instantiated, the same way the action helper does. It would be also helpful for refactoring.

What do you think about that?