Login error fix

Fix for [TNL140]: Studio's signin page appears unresponsive after a user submits one incorrect login.  If further logins were also incorrect, the error message remained unchanged, which could give the impression that the server was no longer responsive. 

Per the comments in the original Jira ticket, I have addressed this by making the error message invisible if the user edits either the username or password fields.  I've also provided a Lettuce test that verifies the new behavior.

Feedback is appreciated; I am new to this codebase.  

This contribution is covered under the OpenCraft contributor agreement.  Attn: @antoviaque 

[TNL140]: https://openedx.atlassian.net/browse/TNL-140
Why do you do the if, then assert false? Why not just
```python
def i_should_not_see_an_error_message(step):
    assert_false(world.css_visible('div#login_error'))
```