curl_multi_strerror(3) on macOS

The text 585594f6aafb, as macOS 10.12.5, macOS 10.12.4, macOS 10.12.3, macOS 10.12.2, macOS 10.12.1, macOS 10.12 shipped it.

return string describing error code

curl_multi_strerror(3) on macOS now