Username availability checker breaks inline validation
Problem
The username field on the registration form's availability checker breaks inline validation on the username field.
Proposal
Improve the username availability checker in order to make compatible with other validation scripts and thus improve the user experience.
Steps to reproduce
There are two ways to reproduce this bug:
Incorrect input first
1.1 Input a incorrectly formatted username, eg "
1.2 Input any username that is unavailable or available, eg "Foo", and now the validations messages no longer displays.
Unavailable username first
2.1 Input any username that is unavailable or available, eg "Foo", and the validation message displays.
2.2 Input a incorrectly formatted username, eg "