throws error for missing docs in DocTest

This fixes issue #2482.

Now the code in https://groups.google.com/forum/#!topic/elixir-lang-talk/BWnyrCRaqm4 referred in the issue will get an error message when module DNA is not compiled, and when module DNA is compiled we will have one passing test.
Change "MODULENAME" to the actual module (`#{module}`).

You can remove "Or" from the second sentence.