[bug] #33287 Contact creator wrong error behavior

http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33287&start=0
To test: enable the contactcreator plugin and choose a category 
When creating a user in frontend "Create new account" or backend and the user name is the same as an already existing Contact alias in the same category as defined in Contactcreator, the user may be created but the error is picked from com_contact and produces a false error as the user has been created.
Back-end:
![screen shot 2014-02-17 at 07 38 08](https://f.cloud.github.com/assets/869724/2182890/36b2e1ba-979e-11e3-83bc-70b262c53335.png)

Frontend:
![screen shot 2014-02-17 at 07 40 59](https://f.cloud.github.com/assets/869724/2182893/82d26232-979e-11e3-8738-b8e92efb3729.png)

This PR corrects the behavior and display a correct message.