Improve error message for IEx.Helpers.r when module does not exist.

Before this commit, when trying to reload an unloaded module, the
user would see an error like:

iex(1)> r InexistentModule
** (UndefinedFunctionError) undefined function: InexistentModule.module_info/1
    InexistentModule.module_info(:compile)
    lib/iex/lib/iex/helpers.ex:352: IEx.Helpers.source/1
    lib/iex/lib/iex/helpers.ex:311: IEx.Helpers.do_r/1
    lib/iex/lib/iex/helpers.ex:304: IEx.Helpers.r/1

After this commit, the user should see


iex(1)> r InexistentModule
** (ArgumentError) Unloaded module Elixir.InexistentModule. Try loading the module's beam file with `IEx.Helpers.l\1`
    lib/iex/lib/iex/helpers.ex:312: IEx.Helpers.do_r/1
    lib/iex/lib/iex/helpers.ex:304: IEx.Helpers.r/1

Which is more informative.

Author: Ricardo de Cillo

lib/iex/lib/iex/helpers.ex

@@ -308,6 +308,10 @@ defmodule IEx.Helpers do
   end
 
   defp do_r(module) do
+    unless :code.is_loaded(module) do
+      raise ArgumentError, message: "Unloaded module #{module}. Try loading the module's beam file with `IEx.Helpers.l\\1`"
+    end
+
     source = source(module)
     cond do
       source == nil ->

lib/iex/test/iex/helpers_test.exs

@@ -305,7 +305,7 @@ defmodule IEx.HelpersTest do
   end
 
   test "r helper unavailable" do
-    assert_raise UndefinedFunctionError, "undefined function: :non_existent_module.module_info/1", fn ->
+    assert_raise ArgumentError, "Unloaded module non_existent_module. Try loading the module's beam file with `IEx.Helpers.l\\1`", fn ->
       r :non_existent_module
     end
   end