Don't try static methods on `lein run`: better error msg

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.