tutorial: explain how to find origin of exception
tutorial: explain how to find origin of exception

explain throw_with_backtrace() and make_exception_future_with_backtrace()
and they can be used to understand where an exception was thrown.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20180523160618.4838-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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