DisabledBackend now shows a warning when celery attempts to store result with it.

When celery backend misconfiguration happens, it is immensly time-consuming to pinpoint why certain functionality stops working properly.

Introducing this warning message (that should show only once per program run) should simplify the process of the configuration debug quite noticeably.

As "warnings" library have been part of Python batteries since version 2.1, this change should be compatible with most setups out there.