Handling http errors from HTTPoison / Hackney

I’m attempting to gracefully handle failures from an api call and am struggling a bit with how to do this. Take for example the following:

    case HTTPoison.get("https://url.com") do
      {:ok, %HTTPoison.Response{status_code: 200, body: body}} ->
        ...
      {:error, %HTTPoison.Error{} = error} ->
        ...
    end

Works great for one real world test case. What I’m struggling with is two things:

1. Under what circumstances does HTTPoison return :error vs :ok? I can’t find this anywhere. I don’t know if it’s possible that it would ever return :ok with a status code of 500, or if it will always return :error on any status code > 400 or what… Do I need to match on both :error as well as :ok with a status code > 400 to catch failures?
2. I cannot for the life of me figure out how to test this. We’re using Bypass in our tests, and I can have it return a Plug.Conn.resp with a 500 status code but the actual response still comes back with an :ok and not :error. I cannot for the life of me figure out how to get Bypass to return an :error response.

Any help is greatly appreciated!