Fixed in in rev5976:71c68961d9f4.

The code to allow the admin to enable/disable username collisions at 
registration time (rather than confirmation) has a set of tests.

The code to pretty up the error message doesn't.
Also it's a little fragile as it overrides a specific ValueError 
exception message. If that message changes the friendlier message will 
not be displayed.

Roundup AFAICT has no way to trigger a detector since that requires a 
'commit to database' flow that only happens on confirmation. At 
registration time a small set of data is stored in the OTK database 
that doesn't trigger detectors.

This change at least fixes the primary complaint.