Introducing a new Error for watch exception

[GustavoCaso]

I noticed that the python redis library will raise an exception when you are watching any key.

In my opinion, think is quite useful when you are inside loops and using watch method.

Here is an example:

  while Time.now.to_i < threshold do
    begin
      client.watch("market:", buyer) do
        price = client.zscore("market:", item).to_i
        funds = client.hget(buyer, 'funds')

        if price != seller_price || price > funds
          client.unwatch
          break
        end

        client.multi do |multi|
          multi.hincrby(seller, 'funds', price)
          multi.hincrby(buyer, 'funds', price * -1)
          multi.sadd(inventory, itemid)
          multi.zrem('market:', item)
        end

        break
      end
    rescue Redis::WatchError
      next
    end
  end

I would like to know if you think it something useful and if so I do not mind working on a PR to add it