t/one-core-peak: Improving check_sysblock_value error handling

t/one-core-peak: Improving check_sysblock_value error handling

The current code was reporting the following output:
	cat: /sys/block/nvme0n1/queue/wbt_lat_usec: Argument invalide
	nvme0n1: /sys/block/nvme0n1/queue/wbt_lat_usec set to 0.
	Warning: nvme0n1: Cannot set 0 on /sys/block/nvme0n1/queue/wbt_lat_usec

This is problematic for several reasons:
- cat reports an error at reading wbt_lat_usec
- a message says it set wbt_lat_usec to 0
- a warning reports it cannot set wbt_lat_usec to 0

This commit:
- prevents the first error to be printed
- only report wbt_lat_usec is set to 0 if succeed unles it print the Warning message.

Signed-off-by: Erwan Velu <erwanaliasr1@gmail.com>