My 2 cents
Checking for the existence of a user name during a sign up process is good to have. But it's not a good idea for a login form. Most logins you see on websites just tell you that the login failed, and give you options to retrieve you info.
For security reasons, if you tell the user which one of the fields doesn't validate, it just makes it that much easier for someone to hack into the account.
Plus it makes your job easier as this is the default way the login behaviors work. Just something to think about.