Nicer exception message when .xaml != Resource

I'm also trying to give more descriptive error message for when some
.xaml files are Resource and some are not.  Still learning F#!
Added comments and defined TODOs

Removed unused/practice code to prepare for PR.  I added TODO's with
what my intent is to provide better exception messages during failure at
runtime.
Updated PR

Excluding comments
Merge pull request #10 from kevlut/master

Provided a more detailed exception message when .xaml files aren't set to Resource