I can't find any hint of the Reject exception that is supposed to 
exist.

Using:

       raise Reject, "You can't resolve this 
issue because
   dependedon issue(s) " +  " ".join(unresolved) 
+
   " must be resolved first."

results in an error unknown 
global Reject. Using:

       raise "Reject", "You can't resolve 
this issue because
   dependedon issue(s) " +  " 
".join(unresolved) + 
   " must be resolved first."

works, 
but I am not sure it is right. Is there a standard list of exceptions that 
can/should be thrown from detectors?

The Reject is talked 
about in the design document and seems to make more sense 
than the ValueError or roundupdb.DetectorError in the 
customization document.