pool: Add missing response to subscribe handler.

The handler for subscribe requests seemed to have incomplete error
handling for the case where FetchMinerDifficulty failed. No response was
being returned to the caller.

This adds a generic Unknown error response so that the caller is not
left hanging.

pool/client.go

@@ -394,6 +394,9 @@ func (c *Client) handleSubscribeRequest(req *Request, allowed bool) error {
 	info, err := c.cfg.FetchMinerDifficulty(miner)
 	if err != nil {
 		c.mtx.Unlock()
+		sErr := NewStratumError(Unknown, err)
+		resp := SubscribeResponse(*req.ID, "", "", 0, sErr)
+		c.sendMessage(resp)
 		return err
 	}
 	c.miner = miner