[TASK] Rename bg-image (login/error) to avoid browser caching issues

[TASK] Rename bg-image (login/error) to avoid browser caching issues

Once you upgrade to TYPO3 6.0 your browser might still use the old cached
gray gradient background "images/login/background.png" which doesn't fit
the new login screen layout.

By renaming it, we make sure that browser will fetch the new one.

As the file is also being used by the "error page", we move it to a more
generic location (images/backgrounds instead of images/login).

Releases: 6.1, 6.0
Resolves: #43323
Change-Id: I8e5e96112878006a111d1bff37500c699dd279a3
Reviewed-on: http://review.typo3.org/16927
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn