got rid of getattr (not RPython)

After some refactoring the function check_type_errors used
getattr(space, "w_something"). Since this doesn't work in RPython, I got rid of
it using space.getclassfor(space, W_SomethingObject) instead.
input args of check_type_errors changed once again

Now check_type_error works on strs which contain the names of Ruby Types such
as Numeric or Integer. This means that getattr is used again in this function.