It's time to clean up this mess. http://wiki.dlang.org/DIP33
I like the idea, I think some specifics may need worked out (already being discussed)
You're likely purposely avoiding this. But when the subject of a "Exception Hierarchy" comes up thoughts of having specific modules to house the exceptions come up. I don't think this is the primary concern, but my position is to keep the exceptions in their std implementation/use.