Fix ExUnit.MultiError.message/1 implementation

Before, the function would always fail with ProtocolUndefinedError, since
the exception struct itself is not enumberable.

Author: michalmuskala

lib/ex_unit/lib/ex_unit/assertions.ex

@@ -30,9 +30,9 @@ defmodule ExUnit.MultiError do
 
   defexception [errors: []]
 
-  def message(exception) do
+  def message(%{errors: errors}) do
     "got the following errors:\n\n" <>
-      Enum.map_join(exception, "\n\n", fn {kind, error, stack} ->
+      Enum.map_join(errors, "\n\n", fn {kind, error, stack} ->
         Exception.format_banner(kind, error, stack)
       end)
   end