fix wrong error message for `nil.func()` call

The expected behavior for `nil.func()` should be:

    iex(1)> nil.test()
    ** (UndefinedFunctionError) undefined function: nil.test/0
       nil.test()

Also via cli `elixir -e`:

     $ elixir -e 'nil.test()'
     ** (UndefinedFunctionError) undefined function: nil.test/0
     nil.test()
     ....

Undefined function calls behave like:

    iex(1)> hello()
    ** (RuntimeError) undefined function: hello/0