iotune: throw exception if no data was collected
iotune: throw exception if no data was collected

Currently it is possible that io_worker::get_io_rates divides by zero
resulting in bogus iops/bytes_per_sec: 9223372036854775808, i.e. 0x8000000000000000

Refs scylladb/scylla#4024

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20190101194546.13206-1-23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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