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
`world.css_visible()` unconditionally asserts that the given element is present.  Before an unsuccessful login, no `login_error` element exists, and the simpler test you suggest fails with a thrown exception instead of returning `False`.