net.connect (with timeout), raise error on connect failure

Under Linux (probably POSIX), the current code tests for timeout, but
does not test for connection failure. connectAsync() returns succesfully
upon an EINPROGRESS result; but at this point, the connection state is
still unknown. After selectWrite() is done, we need to test the socket
for errors again.

lib/pure/net.nim

@@ -1664,6 +1664,9 @@ proc connect*(socket: Socket, address: string, port = Port(0),
   if selectWrite(s, timeout) != 1:
     raise newException(TimeoutError, "Call to 'connect' timed out.")
   else:
+    let res = getSockOptInt(socket.fd, SOL_SOCKET, SO_ERROR)
+    if res != 0:
+      raiseOSError(OSErrorCode(res))
     when defineSsl and not defined(nimdoc):
       if socket.isSSL:
         socket.fd.setBlocking(true)