Improve error handling

[rvdsteege]

In internal Help Scout ticket https://secure.helpscout.net/conversation/2751009728/27927 a user informed about an unclear error message:

Unknown response from Pay.nl error.

It appears the given API token is invalid and this results in a HTTP 401 Unauthorized with an empty response body. We could improve the error message a bit for these cases.

wp-pronamic-pay-pay-nl/src/Client.php

Lines 100 to 114 in 9afee62

	// Body.
	$body = wp_remote_retrieve_body( $response );
	$result = json_decode( $body );
	// Result is array.
	if ( is_array( $result ) ) {
	return $result;
	}
	// Result is object
	// NULL is returned if the json cannot be decoded or if the encoded data is deeper than the recursion limit.
	if ( ! is_object( $result ) ) {
	throw new \Exception( __( 'Unknown response from Pay.nl error.', 'pronamic_ideal' ) );
	}