Display an error when trying to create a user which already exists

Steps to reproduce:

1. create a first user whose username is jdoe
2. go back to the user creation form
3. fill the fields and use the same value "jdoe" for the username
4. click on the Create button and observe that no error is displayed

The problem can be observed either when
1) you do not define a password. In this case,

• you have this message in the browser console

  POST http://localhost:8080/nuxeo/api/v1/automation/User.Invite 500
  Operation request failed: Error
  

• you have this error in the server logs

  ERROR [WebEngineExceptionMapper] org.nuxeo.ecm.platform.usermanager.exceptions.UserAlreadyExistsException: Failed to invoke operation: User.Invite, Failed to invoke operation User.Invite, null
  org.nuxeo.ecm.platform.usermanager.exceptions.UserAlreadyExistsException: Failed to invoke operation: User.Invite, Failed to invoke operation User.Invite, null
  

  => the error HTTP 500 is now replaced by a HTTP 409 thanks to NXP-29793

2) you define a password: In this case, only an error is displayed in the browser console

POST http://localhost:8080/nuxeo/api/v1/user 409
Resource request failed: User already exists

Expected behavior: in both cases, a message should warn the current actor that the user already exists.