Rename Sink::SinkError to Sink::Error

[seanmonstar]

Closes #1707

[cramertj]

r=me with travis passing.

[seanmonstar]

Hrm, I'm guessing this failure is due to improvements in nightly?

---- join_size stdout ----
thread 'join_size' panicked at 'assertion failed: `(left == right)`
  left: `32`,
 right: `44`', futures/tests/async_await_macros.rs:178:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

[taiki-e]

@seanmonstar Yes, that is because the generator size has been improved. (rust-lang/rust#61922).

[cramertj]

Yeah, I've got a PR updating those.

[cramertj]

#1711

[cramertj]

@seanmonstar Would you mind rebasing just so I can see a clean CI run on this before merging?