Karel said he had asked their developers for a list of the codes and was going to get back to me when he heard from them. However, I haven't seen anything yet. It probably just slipped their mind.
Usually the way it works is that a blinking red LED means that an error occured. In addition to the blinking red, there is also a combination of some of the other green LEDs. The specific combination of green LEDs is what tells you what the error was.
The only one I've been able to identify so far is the "battery too low" error. That was based on a user's feedback from talking with iRex tech support. The red LED was blinking and the bottom two green LEDs on the right hand side were on.