fix calls to get_error_message()

When trying to install a couple of the recommended plugins I was receiving 503 errors on the ajax request.

The fix is to call $failed_request->get_error_message() instead of get_error_message( $failed_request ) since $failed_request is an instance of WP_Error.

For reference, my debug log had these two entries:

PHP Fatal error:  Call to undefined function get_error_message() in /path-to-wp-content/plugins/developer/developer.php on line 347
PHP Fatal error:  Call to undefined function get_error_message() in /path-to-wp-content/plugins/developer/developer.php on line 363