Added detection of curl_exec failure

[mTorres]

I've hit a little issue while using curl on windows due to SSL verification failure. I've discovered what happened after checking the curl_exec returned value and I've thought this little modification might be useful to have.

[asimlqt]

This is common problem on windows, there is a solution post on stackoverflow: http://stackoverflow.com/a/16495053/1993689

[colinfrei]

This merge request would be helpful in debugging other curl errors as well - I'd recommend reopening this and merging these changes.