Return (Result<(),E>,) in drain_the_queue

[est31]

This refactors the drain_the_queue function to return a
tuple containing Result. That way, it's still not
possible to use ? or try! to handle errors,
but for readers of the function declaration it's
clearer now that the error actually indicates one.

Bonus: it makes the calling code of drain_the_queue simpler.

[rust-highfive]

r? @alexcrichton

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[est31]

I got the idea for this while working on #8437 as I was wondering how to return something in the case no error happened. It's okay if you reject this PR now, I can also include it as a commit in #8437.

[alexcrichton]

I personally prefer the previous iteration, I think in general I just find working with 1-tuples pretty weird.

[est31]

Ok I'll include it in #8437 instead, where I'm putting a value into the happy path of the Result, so the option will cease to be viable.