Update OAuth2.Client.get_token! errors

[chrislaskey]

Updates the OAuth2.Client.get_token! function to handle error OAuth2.Response structs. I sent some incomplete data to Facebook Graph and saw this error bubble up:

[error] %ArgumentError{message: "raise/1 expects a module name, string or exception as the first argument, got: %OAuth2.Response{body: %{\"error\" => %{\"code\" => 101, \"fbtrace_id\" => \"Eelth+pfiU9\", \"message\" => \"Missing client_id parameter.\", \"type\" => \"OAuthException\"}}, headers: [{\"www-authenticate\", \"OAuth \\\"Facebook Platform\\\" \\\"invalid_client\\\" \\\"Missing client_id parameter.\\\"\"}, {\"access-control-allow-origin\", \"*\"}, {\"pragma\", \"no-cache\"}, {\"cache-control\", \"no-store\"}, {\"x-fb-rev\", \"3349395\"}, {\"content-type\", \"application/json\"}, {\"x-fb-trace-id\", \"Eelth+pfiU9\"}, {\"facebook-api-version\", \"v2.8\"}, {\"expires\", \"Sat, 01 Jan 2000 00:00:00 GMT\"}, {\"x-fb-debug\", \"9GWPCrYAIzlrSnt0iLqP/3E4hdmOJpP7QXxBOXTG2Ew7CloqVwka+HXAjyCTewfTaH0F/sUVkDLMcm7+6jOEEQ==\"}, {\"date\", \"Thu, 05 Oct 2017 15:36:54 GMT\"}, {\"connection\", \"keep-alive\"}, {\"content-length\", \"114\"}], status_code: 400}"}

Using OAuth2.Request.request! as a reference, I added a similar error handling case to OAuth2.Client.get_token!.

Now a proper error is raised with pretty printed debugging information:

[error] #PID<0.1063.0> running ExampleOAuth2Web.Endpoint terminated
Server: oauth2.example.dev:80 (http)
Request: POST /
** (exit) an exception was raised:
    ** (OAuth2.Error) Server responded with status: 400

Headers:

www-authenticate: OAuth "Facebook Platform" "invalid_client" "Missing client_id parameter."
access-control-allow-origin: *
pragma: no-cache
cache-control: no-store
x-fb-rev: 3349509
content-type: application/json
x-fb-trace-id: CIc8eDTN/BC
facebook-api-version: v2.8
expires: Sat, 01 Jan 2000 00:00:00 GMT
x-fb-debug: wJzExwqDXtAkHKtBNo4xKE21zBO730qctNXaP9A7bqenLRuTkGq109qvx8iOy063OiE3uVunf/Mt1trqvhuwVg==
date: Thu, 05 Oct 2017 15:40:21 GMT
connection: keep-alive
content-length: 115

Body:

%{"error" => %{"code" => 101, "fbtrace_id" => "CIc8eDTN/BC", "message" => "Missing client_id parameter.", "type" => "OAuthException"}}

        (oauth2) lib/oauth2/client.ex:249: OAuth2.Client.get_token!/4

[coveralls]

Coverage decreased (-2.4%) to 92.118% when pulling a4bc623 on chrislaskey:get-token-error into 246897a on scrogson:master.

[coveralls]

Coverage decreased (-2.4%) to 92.118% when pulling a4bc623 on chrislaskey:get-token-error into 246897a on scrogson:master.

[coveralls]

Coverage decreased (-0.4%) to 94.089% when pulling e46dd8d on chrislaskey:get-token-error into 246897a on scrogson:master.

[coveralls]

Coverage increased (+0.6%) to 95.074% when pulling fa3c6a0 on chrislaskey:get-token-error into 246897a on scrogson:master.

[scrogson]

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