Fix exunit format_message crash when message is not a string

[lukaszsamson]

I noticed this crash in the logs. I'm not sure if a struct is supposed to be in message. The ExUnit.AssertionError struct typespec has type any

an exception was raised:
    ** (FunctionClauseError) no function clause matching in ExUnit.Formatter.pad_multiline_2
USER_PATH_ex_unit_formatter.ex:735: ExUnit.Formatter.pad_multiline(#Ecto.Changeset<action: nil, changes: %{foo_id: 123}, errors: [prefered_locale: {"can't be blank", [validation: :required]}], data: #Foo.Domain<>, valid?: false, ...>, 5)
USER_PATH_ex_unit_formatter.ex:616: ExUnit.Formatter.format_message_2
USER_PATH_ex_unit_formatter.ex:377: ExUnit.Formatter.format_exception_6
USER_PATH_ex_unit_formatter.ex:278: anonymous fn_7 in ExUnit.Formatter.format_test_failure_5
        (elixir 1.18.2) <REDACTED: user-file-path>:1815: anonymous fn_2 in Enum.map_join_3
        (elixir 1.18.2) <REDACTED: user-file-path>:4493: Enum.map_intersperse_list_3
        (elixir 1.18.2) <REDACTED: user-file-path>:1815: Enum.map_join_3
USER_PATH_ex_unit_formatter.ex:277: ExUnit.Formatter.format_test_failure_5

[josevalim]

Please rebase or change of branches! Thanks!

[josevalim]

Thank you, can we have a test? We already have a suite that verifies how failures and, although a message is always meant to be a string, I agree calling inspect as fallback is good. Here is a good candidate for a unit test: https://github.com/elixir-lang/elixir/blob/main/lib/ex_unit/test/ex_unit/formatter_test.exs