drush convert|verify makefile don't respond properly to including core or disallowed project types in the .make file

attached patch refactors the convert/verify functionality a bit, so that it responds correctly when a either drupal core or a disallowed project type (like a profile, for example) are included in the .make file.

this has been tested and works well. i do think, in the long run, we should really look at refactoring the entire makefile validation approach in drush_make -- if it was build from the ground up via objects, i think there would be less duplication of code.