lib/curl: Only check individual request errors

lib/curl: Only check individual request errors

It looks like `curl_multi_socket_action()` will return an error
if *one* of the requests has an error, but we already check
for that explicitly by iterating over each handle.

In libcurl, the "easy" layer doesn't really make use of this
return value.  I did a bit of looking elsewhere; systemd
does check it as a runtime error, not an assertion.  librepo
doesn't use the multi interface.

Closes: https://github.com/ostreedev/ostree/issues/1035
Closes: #1038
Approved by: jlebon