Improve configuration error messages (#27)

* Get Github config with Application.fetch_env!

Gives a bit more of a useful error message if the user has messed
up and not added that config correctly, as opposed to "no function
clause matching in Keyword.merge/2".

* Checking configuration for required keys

Checks for the presence of client_id and client_secret. This may be a
step to far.

* Code style change

Author: paulanthonywilson

lib/ueberauth/strategy/github/oauth.ex

@@ -28,7 +28,12 @@ defmodule Ueberauth.Strategy.Github.OAuth do
   These options are only useful for usage outside the normal callback phase of Ueberauth.
   """
   def client(opts \\ []) do
-    config = Application.get_env(:ueberauth, Ueberauth.Strategy.Github.OAuth)
+    config =
+    :ueberauth
+    |> Application.fetch_env!(Ueberauth.Strategy.Github.OAuth)
+    |> check_config_key_exists(:client_id)
+    |> check_config_key_exists(:client_secret)
+
     client_opts =
       @defaults
       |> Keyword.merge(config)
@@ -73,4 +78,14 @@ defmodule Ueberauth.Strategy.Github.OAuth do
     |> put_header("Accept", "application/json")
     |> OAuth2.Strategy.AuthCode.get_token(params, headers)
   end
+
+  defp check_config_key_exists(config, key) when is_list(config) do
+    unless Keyword.has_key?(config, key) do
+      raise "#{inspect (key)} missing from config :ueberauth, Ueberauth.Strategy.Github"
+    end
+    config
+  end
+  defp check_config_key_exists(_, _) do
+    raise "Config :ueberauth, Ueberauth.Strategy.Github is not a keyword list, as expected"
+  end
 end