futures_test: Extend nested_exception test
futures_test: Extend nested_exception test

It now also covers function objects that return an exceptional future.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200709213238.3485156-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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