Ability to disable triggering controller action

This is a very small feature, but something I came across recently. To be as unobtrusive as possible, I like to use the `controller_predispatch` events to handle some incoming logic. For example, in the shopping cart, on the last step (review), let's say the customer wants some custom fields added there, and error checking needs to be a part of that. If I can handle the error checking there, and, if there is an error, set the `FLAG_NO_DISPATCH_ACTION`, so it disables the instantiation of the saveOrder action. I can then output some error json, informing the customer that their entry was invalid.