Ah, see, this was what I wanted to avoid.  We don't want to tell people that an administrator login exists--too much info.  Just like you don't tell people if the username or password failed, just that you weren't able to log them in.

Could you change this to something more generic?  Like:   
`"The specified username is unavailable. Please choose a new username and try again."`

I would also recommend using the full error from before, for two reasons:
1. It's a translated string; changing it in any way simply breaks translations.
2. The old error message was more informative :)

Lastly, could you wrap your error in the _() function?  That will make it translatable too!