Have SnowflakeConn.Close() return errors

Return an error if the connection was already closed. On the first
close, return an error if any of the calls inside Close() returned an
error in this order:
- smux.Stream.Close()
- pconn.Close()
- smux.Session.Close()

client/lib/snowflake.go

@@ -243,14 +243,19 @@ type SnowflakeConn struct {
 //
 // The collection of snowflake proxies for this connection is stopped.
 func (conn *SnowflakeConn) Close() error {
+	var err error
 	log.Printf("---- SnowflakeConn: closed stream %v ---", conn.ID())
-	conn.Stream.Close()
+	err = conn.Stream.Close()
 	log.Printf("---- SnowflakeConn: end collecting snowflakes ---")
 	conn.snowflakes.End()
-	conn.pconn.Close()
+	if inerr := conn.pconn.Close(); err == nil {
+		err = inerr
+	}
 	log.Printf("---- SnowflakeConn: discarding finished session ---")
-	conn.sess.Close()
-	return nil // TODO: return errors if any of the above do
+	if inerr := conn.sess.Close(); err == nil {
+		err = inerr
+	}
+	return err
 }
 
 // loop through all provided STUN servers until we exhaust the list or find