Throw meaningful exceptions on missing templates

Lately I have been diving into Marionette and one of the things that I found very inconvenient is missing or forgotten templates. Either wrong template definitions, or just forgetting the template attribute on the view definition, and so on.

Now, currently when I have a missing template attribute I am presented with a backtrace like this:

![missingTpl](https://f.cloud.github.com/assets/16446/348887/f474eb54-9f91-11e2-87c4-9bbc86cf9c6b.png)

The last line is something that has to with my application. Also the error message was as helpful as it might be:

    'undefined' is not a function

I thought - since the renders pass through the Renderer maybe we can shorten this backtrace a bit.