Fix order errors flash

This branch fixes a problem where a blank error message is displayed on the checkout process when an association in a `Spree::Order` is invalid and prevents `Spree::Order#next` from succeeding.

Here is what the actual error message looks like:

![screen shot 2013-09-11 at 11 52 22 am](https://f.cloud.github.com/assets/133/1128588/332780d4-1b6e-11e3-935f-04abbb7fb7a4.png)

In most circumstances, only the base error is displayed, but there are cases where the state of the order is made invalid due to a bug ([like one I found with spree_active_shipping](https://github.com/spree/spree_active_shipping/pull/131)) and a nicer error message would make it more obvious.