Change writeTo failed log level to info

In #252 decide to ignore the writeTo failed error with a log,
application can choose callback and query state for the connection
state. But if application keep sending packet during try icerestart
to recover from ICEFailed, the warn log is annoying since the error
is not critical and no need to process. So change it to info.

candidate_base.go

@@ -300,7 +300,7 @@ func (c *candidateBase) close() error {
 func (c *candidateBase) writeTo(raw []byte, dst Candidate) (int, error) {
 	n, err := c.conn.WriteTo(raw, dst.addr())
 	if err != nil {
-		c.agent().log.Warnf("%s: %v", errSendPacket, err)
+		c.agent().log.Infof("%s: %v", errSendPacket, err)
 		return n, nil
 	}
 	c.seen(true)