When creating a user with a username that already exist instead of a validation message user is shown an Internal server error page.

Steps to reproduce:
1. Login as admin
2. Go to Admin -> Users
3. Select "Administrator users"
4. Create new User
5. Fill necessary fields and set "admin" as username
6. Click "Create"

Expected:
User is informed (in a nice way) that the username is already taken

Actual:
User is redirected to error page ("It's an error 500, an internal server error. The server was unable to complete your request and it's not your fault.").
Notification is visible: Argument 'userCreateStruct' is invalid: User with provided login already exists