I used to allow each quote method to set an error flag that would prevent the customer from continuing checkout. Unfortunately, it had been done badly, and it was preventing customers from selecting a quote even if there were some available. Now the error message shows up only if all quotes fail.

I do like the idea of individual error messages for quotes, though. I'll add them in so that they are displayed, but probably not affect the checkout process. Should the error message replace the "notes" field that was recently added, or should they be displayed side-by-side?

I also want to make the super error message customizable. It hasn't even been a translatable string, though if I make it configurable, it won't ever be.