Currently we don't check data_file_directories existance before running iotune,
therefore it's shows unclear error message.
To make the message better, check the directory existance on scylla_io_setup.
Fixes#3137
Signed-off-by: Takuya ASADA <syuu@scylladb.com>
Message-Id: <1517200647-6347-1-git-send-email-syuu@scylladb.com>