handle invalid for test failure

[michaelst]

I was running into an issue with test failures causing the report to fail to write with this error

15:38:27.110 [error] GenServer #PID<0.926.0> terminating
** (MatchError) no match of right hand side value: {:error, :badarg}
    (junit_formatter 3.4.0) lib/formatter.ex:164: JUnitFormatter.handle_suite_finished/1
    (junit_formatter 3.4.0) lib/formatter.ex:73: JUnitFormatter.handle_cast/2
    (stdlib 6.2.2) gen_server.erl:2371: :gen_server.try_handle_cast/3
    (stdlib 6.2.2) gen_server.erl:2433: :gen_server.handle_msg/6
    (stdlib 6.2.2) proc_lib.erl:329: :proc_lib.init_p_do_apply/3

The piece in the report that was causing this I traced back to

[
  ~c"<",
  ~c"error",
  [
    [~c" ", ~c"message", ~c"=\"",
     ~c"Invalid module Elixir.ExUnit.TestModule", ~c"\""]
  ],
  ~c">"
], # followed by a long binary

I'm not sure if there is a reason to treat invalid this way, but this change does allow my report to generate with the correct failure now instead of crashing