Improve `rack.after_reply`

[casperisfine]

Things to do:

• Don't trust that callbacks don't raise. If one raise, swallow the error and continue calling the next one.
• This capability was added to the rack spec under another name: https://github.com/rack/rack/issues/1777 / https://github.com/rack/rack/pull/1952, might be worth aliasing the env key.