future: Make set_to_current_exception public
future: Make set_to_current_exception public

It is just an optimized version of
set_exception(std::current_exception()), so making it public doesn't
expose any internals.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200731000512.468153-2-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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