Illegal password error message displays the password in plaintext

(Bug, Closed -> Fixed, Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
Security
Summary: Reported by Conor: the system rejected his password, and showed it on the screen. That clearly needs to be fixed.
The password was illegal for being too short.
This bug is probably in the Play framework itself, so I may have to rework how we handle this length sanity-check.