Support multiple errors in Promotion Handlers

[kennyadsl]

At the moment, applying a promotion coupon (but I think it is an issue present also in the other promotion handlers) consider a single error possible.

Setting the error happens through setting an error attribute on the class, along with a status_code.

The problem is where we have multiple errors for the same promotion, for example, if the coupon code is added to an order that does not meet multiple promotion rules for that promotion, as discussed here.

A possible solution

I think we should change error to errors adding a new class that represent that PromotionHandlerError concept, or something similar. We should also try to deprecate the current usage of error and status_code printing a deprecation warning and falling back to the first PromotionHandlerError associated with that PromotionHandler.

cc @fkoessler