raise Unicorn::ClientShutdown if client aborts in TeeInput
Leaving the EOFError exception as-is bad because most
applications/frameworks run an application-wide exception
handler to pretty-print and/or log the exception with a huge
backtrace.
Since there's absolutely nothing we can do in the server-side
app to deal with clients prematurely shutting down, having a
backtrace does not make sense. Having a backtrace can even be
harmful since it creates unnecessary noise for application
engineers monitoring or tracking down real bugs.