Disabled alias field on specific error, while creating new user.

Steps to reproduce:

1. Navigate to users page, then click "Create user"
2. Set alias "guest" and fill out other mandatory fields.
3. Submit.
4. Observe: Error "User with alias "guest" already exists."

Result:
"Alias" input field is disabled now, there is no way to update it's value.
Expected:
Being able to proceed.