Fix error detection

The previous error detection code seems to have been copied from stevenmaguire/oauth2-microsoft, but the response format for errors seem to be different when using Azure. 
Part of an example JSON:
{
    "error": "invalid_client",
    "error_description": "[…]",
…

src/Provider/Azure.php

@@ -87,7 +87,7 @@ protected function checkResponse(ResponseInterface $response, $data)
     {
         if (isset($data['error'])) {
             throw new IdentityProviderException(
-                (isset($data['error']['message']) ? $data['error']['message'] : $response->getReasonPhrase()),
+                (isset($data['error_description']) ? $data['error_description'] : $response->getReasonPhrase()),
                 $response->getStatusCode(),
                 (string)$response->getBody()
             );