Add 4XX errors body to exception

[djagya]

Usually errors 4XX have useful explanation about an occurred error, but the library does parse and return error body only if error code is < 400.

See ApiResource line 194.

I think it's better to do something like

  // Handle 4XX responses, 5XX responses will throw an HttpAdapterException
if ($statusCode < 500) {
    $body = json_decode($response->getBody()->getContents(), true);
    if ($statusCode < 400) {
        return $body;
    } elseif ($statusCode === 404) {
      throw new APIResponseException('The specified resource does not exist', 404);
    } else {
        throw new APIResponseException('Received bad response from ' . ucfirst($this->endpoint) . ' API: '. $statusCode.', body:' . print_r($body));
    }
}

I'm ready to prepare a PR.

[kieldoyle]

+1

Getting back a message like this:

Received bad response from Transmissions API: 422

Is pretty useless as the status code covers many errors as illustrated here:
https://support.sparkpost.com/customer/en/portal/articles/2140916-extended-error-codes

Sparkpost's actual response body is really useful for debugging
{ "errors": [ { "message": "required field is missing", "description": "At least one of 'text' or 'html' needs to exist in 'content'", "code": "1400" } ] }

[chandon]

+1

[RicardoRamirezR]

@zaporylie Nice!!!

[zaporylie]

@treserres You're welcome