osl: Remember the last error before returning osl_Socket_Error

This prevents from e.g. com.sun.star.bridge.UnoUrlResolver's
emitting confusing messages, when trying to connect to a port
on which no LibreOffice process is listening, like:
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
>  __main__.NoConnectException: Connector : couldn't connect to socket (Success)
                                                                        ^^^^^^^
After applying this patch:
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
>  __main__.NoConnectException: Connector : couldn't connect to socket (Connection refused)

You can see the above behavior with the following python code:

import uno
x = uno.getComponentContext()
y = x.ServiceManager
z = y.createInstanceWithContext("com.sun.star.bridge.UnoUrlResolver", x)
url = "uno:socket,host=localhost,port=2002;urp;StarOffice.ComponentContext"
a = z.resolve(url)

... provided that no process is waiting on port 2002.

Change-Id: Id094cf9271fe77d84f2284d91a0e452448de2bc2
Reviewed-on: https://gerrit.libreoffice.org/52018Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

sal/osl/unx/socket.cxx

@@ -1401,7 +1401,6 @@ oslSocketResult SAL_CALL osl_connectSocketTo(oslSocket pSocket,
     fd_set   ExcptSet;
     int      ReadyHandles;
     struct timeval  tv;
-    oslSocketResult Result= osl_Socket_Ok;
 
     SAL_WARN_IF( !pSocket, "sal.osl", "undefined socket" );
 
@@ -1490,13 +1489,19 @@ oslSocketResult SAL_CALL osl_connectSocketTo(oslSocket pSocket,
             nSockOpt = getsockopt ( pSocket->m_Socket, SOL_SOCKET, SO_ERROR,
                                     &nErrorCode, &nErrorSize );
             if ( (nSockOpt == 0) && (nErrorCode == 0))
-                Result = osl_Socket_Ok;
+            {
+                osl_enableNonBlockingMode(pSocket, false);
+                return osl_Socket_Ok;
+            }
             else
-                Result = osl_Socket_Error;
+            {
+                pSocket->m_nLastError = (nSockOpt == 0) ? nErrorCode : errno;
+                return osl_Socket_Error;
+            }
         }
         else
         {
-            Result= osl_Socket_Error;
+            return osl_Socket_Error;
         }
     }
     else if (ReadyHandles < 0)  /* error */
@@ -1508,17 +1513,13 @@ oslSocketResult SAL_CALL osl_connectSocketTo(oslSocket pSocket,
             return osl_Socket_Interrupted;
         }
         pSocket->m_nLastError=errno;
-        Result= osl_Socket_Error;
+        return osl_Socket_Error;
     }
     else    /* timeout */
     {
         pSocket->m_nLastError=errno;
-        Result= osl_Socket_TimedOut;
+        return osl_Socket_TimedOut;
     }
-
-    osl_enableNonBlockingMode(pSocket, false);
-
-    return Result;
 }
 
 oslSocket SAL_CALL osl_acceptConnectionOnSocket(oslSocket pSocket,