Try to use Mox to create a mock to handle errors of HTTPoison

config.exs

# config/config.exs
import Config
config :ex_tldr, http_client: HTTPoison

ex_tdlr.ex

# lib/ex_tldr.ex
defmodule ExTldr do
  @moduledoc """
  Documentation for ExTldr, an Elixir client for tldr-pages.
  """

  @http_client Application.get_env(:ex_tldr, :http_client)
  
  # Another functions
  
  defp describe(os, term) do
    case @http_client.get(process_url(os, term)) do
      # Conditions to check
    end
  end
end
  

ex_tldr_test.exs

# test/ex_tldr_test.exs
defmodule ExTldrTest do
  use ExUnit.Case
  import ExUnit.CaptureIO
  alias ExTldr
  alias ClientMock
  doctest ExTldr

  import Mox

  setup :verify_on_exit!
  
  # Several tests
  
  test "should return lack of internet" do
    assert_raise NoInternetConnectionError,
                 fn ->
                   ClientMock
                   |> expect(:get, fn _ -> {:error, %HTTPoison.Error{reason: :nxdomain}} end)
                 end
  end
end

test.exs

# config/test.exs
import Config
config :ex_tldr, http_client: ClientMock

test_helper.exs

# test/test_helper.exs
ExUnit.start()

Mox.defmock(
  ClientMock,
  for: HTTPoison.Base
)