Improve the awareness of the underneath OS/error issue in error messages

When entering a wrong URL (http://10.10.10.10 for instance), the error message is Connection error: please verify URL
In this specific case, the real error underneath is WSAETIMEDOUT (Errno 10060) on Windows.
This error WSAETIMEDOUT can however also be obtained when the server is completely irresponsive and drops requests, while the URL is OK.
In this second case, the error message in the IHM is the same (of course, same symptom client-side) but appears to be misleading as the resolution is not even on client-side.

It is suggested to add in Drive client error messages in the IHM the real OS/network error gotten and associated error message (that Drive writes in nxdrive.log) in a attempt not to mislead the end user.