return a result from sse sender

[jbr]

in order to allow endpoints to break out of loops

dependent on: http-rs/async-sse#3

closes #591

[jbr]

This is now blocked on an async-sse release (which I think is a major release because this changes the api)

[jbr]

Merging this because async-sse 3.0.1 was published, breaking tide master (and tide 0.11). We'll probably want to yank async-sse 3.0.1 and republish as 4.0.0, which will require bumping tide's dep on master to async-sse 4.0.0, but merging this will fix tide master CI

[yoshuawuyts]

@jbr thanks for catching that and running the release!