curl/multi: add default case to print operator
In case a new error code is added on libcurl side without net-cpp being adjusted, the print would stay silent.
This probably won't compile with older libcurl, also the online docs already mention CURLM_UNRECOVERABLE_POLL (12) which isn't in 7.83.1 (what's available on Alpine edge) yet. I guess the alternative is not trying to redefine all errors in this project?