Improve CURL error reporting and handling in the GCS filesystem.

- The main semantics change is that we return immediately if curl_easy_perform doesn't return CURLE_OK. The CURL documentation indicates that it's not ok to fetch info if the curl call failed:
https://curl.haxx.se/libcurl/c/curl_easy_getinfo.html
- LOG errors where we can't return a status. Otherwise return with a status immediately.

PiperOrigin-RevId: 187562481