backend: clarify io scheduler setting error message

backend: clarify io scheduler setting error message

If you know *how* fio tries to confirm that the IO scheduler was
successfully set, then the error message "io scheduler not found" makes
sense. However, if you don't know what fio does to confirm the io
scheduler setting, then the error message is confusing. This patch
modifies the error message to indicate that the selected IO scheduler
could not be set.

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>