Sign up: mistakes in error message text for existing username (English language)

Steps to reproduce:

1. Login to Forum if you were logged out.
2. Select English localization for Forum.
3. Click on "Sign Up" button.
4. Fill out "Username" field with the registered user name (for ex. admin)
5. Click on "Sign Up" button.
6. Overview error message text under "Username" field.
   Actual results:
   User with the username already exists.
   Expected results
   User with this username already exists (no full stop at the end)

Reproduced for error message on both Sign Up pop up window and sign up page http://qa.jtalks.org/jcommune/user/new