Fixing error message when docpad can't find the layout

I was getting the error
    ReferenceError: layoutSelector is not defined
      at ...\node_modules\docpad\out\lib\models\document.js:398:74
      ...

Instead of the more helpful `Error: Could not find the layout service-landing-page on /user-modeling.html` because `layoutSelector` wasn't defined in the `renderLayouts` function.

This fixes that.
Merge pull request #897 from nfriedly/patch-2

Fixing error message when docpad can't find the layout