Fix bad stacktrace formatting

[mitchellhenke]

I've had some pretty bad crashes from this (bringing down the entire verk application), and the root cause is from System.stracktrace here returning a stacktrace like {GenServer, :call, [:my_named_process, \"1\", 5000]}.

Verk eventually attempts to format with Exception.format_stacktrace/1 here, which doesn't work since that calls Enum.map_join, and a tuple isn't Enumerable.

A contrived example of a worker that causes this:

defmodule Verk.TestWorker do
  def perform(key_or_keys) do
    GenServer.call(:my_named_process, "1")
  end
end

I added a test that will fail without my changes:

  1) test call retry on a job with non-enum stacktrace (Verk.QueueManagerTest)
     test/queue_manager_test.exs:213
     ** (Protocol.UndefinedError) protocol Enumerable not implemented for {GenServer, :call, [:process, "1"]}
     stacktrace:
       (elixir) lib/enum.ex:1: Enumerable.impl_for!/1
       (elixir) lib/enum.ex:116: Enumerable.reduce/3
       (elixir) lib/enum.ex:1486: Enum.reduce/3
       (elixir) lib/enum.ex:1118: Enum.map_join/3
       (elixir) lib/exception.ex:388: Exception.format_stacktrace/1
       (verk) lib/verk/queue_manager.ex:134: Verk.QueueManager.build_retry_job/5
       (verk) lib/verk/queue_manager.ex:122: Verk.QueueManager.handle_call/3
       test/queue_manager_test.exs:224

Beyond it's impact on verk here, I'm not sure if it's considered an Elixir issue that System.stacktrace may return something not necessarily formattable.

[coveralls]

Coverage increased (+0.1%) to 81.948% when pulling a71e18d on mitchellhenke:fix-bad-stacktrace-formatting into 7ee6e4d on edgurgel:master.

[edgurgel]

I think we need to review how Elixir deal with stacktraces with ExUnit as they probably need to somehow make the stacktrace readable.

But this is awesome start as people won't have Verk restarting because of this. Thanks for the PR.

[edgurgel]

I'm keen to release a new version this weekend. Thanks again for such great feedback and contributions 👍