curl_multi_strerror(3) on macOS

The text cada2d2eb4bb, as Mac OS X 10.7.5, Mac OS X 10.7.4, Mac OS X 10.7.3, Mac OS X 10.7.2, Mac OS X 10.7.1, Mac OS X 10.7 shipped it.

return string describing error code

curl_multi_strerror(3) on macOS now