Change return type of listen()

[maxbla]

I suggest changing the return type of listen() from (), the unit type to Result<!, ListenError>. This would better document what listen is supposed to do (i.e. never return, unless there was a error setting it up), and it would be more idiomatic Rust. Safe functions in Rust usually don't panic, with some exceptions (like an out of bounds index on an array). This would also require you to create some error type representing possible errors in executing listen.

It would be a breaking change, although pretty easy for users to fix - just change listen() to listen().unwrap() to get the old behavior.

[Narsil]

Fixed in master.

I decided against "!" because it's an experimental feature.