Fixes some user and user-access call errors.

delete-user now 404s if the user doesn't already exist
change-password now 404s if any user doesn't exist
change-password now 400s if any user is invalid
tests added for these three cases
error messages for BadRequests in user calls more specific

fixes bug #1181261

Change-Id: I6c8bcd3d5af99e8c6d49029eaeb26b87a49279d8
Merge "Fixes some user and user-access call errors."