Warn people of unset result backend

[s3rius]

@s3rius Thank you for the quick answer, and it was exactly this. Maybe we should emphasize this in the documentation or put a warning when we use a .wait_for with not Result Backend setup ?

I'm closing this anyway, thanks again!

Originally posted by @sorasful in #426

When the result backend is not specified, wait_for_result will return an empty result without actually waiting for the task to be completed. We should generate a warning or raise an exception to inform users that waiting for a result without a result backend is not possible.

[igor-gorohovsky]

Can I work on this?

[s3rius]

Sure, please do.

[igor-gorohovsky]

Thanks. I'll dig into that this week.