Better error message for require failures

lein run tries to support invoking static methods on classes by
swallowing FileNotFoundExceptions when requiring the namespace to run,
which can obscure the real error when there's a real issue loading a
namespace.

Phil said he wasn't interested in keeping the (undocumented) feature
around, so the easiest fix to the error message problem was to remove
that feature.

Added a test as well.