Improve undefined behaviour warning

Author: José Valim

lib/elixir/lib/module.ex

@@ -1308,7 +1308,7 @@ defmodule Module do
 
         not Code.ensure_compiled?(behaviour) ->
           message =
-            "module #{inspect(behaviour)} does not exist (in module #{inspect(env.module)})"
+            "@behaviour #{inspect(behaviour)} does not exist (in module #{inspect(env.module)})"
 
           unless standard_behaviour?(behaviour) do
             :elixir_errors.warn(env.line, env.file, message)

lib/elixir/test/elixir/kernel/warning_test.exs

@@ -833,7 +833,7 @@ defmodule Kernel.WarningTest do
                @behaviour UndefinedBehaviour
              end
              """)
-           end) =~ "module UndefinedBehaviour does not exist (in module Sample)"
+           end) =~ "@behaviour UndefinedBehaviour does not exist (in module Sample)"
   after
     purge(Sample)
   end