Clearer error message when no framework is provided in the nupkg file

A very small addition to make error messages clearer when no framework dependencies are provided. Even better would be not to fail, but this still helps understand the message :) Otherwise, an `InvalidOperationException` is thrown with the message "Sequence contains no elements" (I had to look into the code to find what it was).

Note that I tried to follow conventions, though there were no tests for error cases, so I didn't add any. I still have to make xunit 2 work on Visual Studio Express...