tutorial: note about asynchronous functions throwing exceptions
tutorial: note about asynchronous functions throwing exceptions

In the "Exceptions vs. exceptional futures" I recommended that
asynchronous functions (functions returning a future) should not
throw, and rather should return an exceptional future.
In this patch, I add a note that this is a recommendation, but
cannot be assumed because even Seastar methods like semaphore::wait()
violate this recommendation.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20180131204930.21850-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

avikivity/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,dreamsxin/seastar,dreamsxin/seastar,cloudius-systems/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,cloudius-systems/seastar,dreamsxin/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar