Warn if result backend is unset

[igor-gorohovsky]

#427

[igor-gorohovsky]

@s3rius Maybe we should add a warning for other task methods if the result backend is not set. What do you think?

[s3rius]

In my opinion the isinstance method is quite expensive to call. We can just add raise in the DummyBackend itself.

Because this isinstance call will be triggered for any ResultBackend. Which might decrease performance.

[s3rius]

I'm wondering. Maybe it'd be better to use warn module to make warnings appear even if you haven't configured logging.