selftests/damon/_damon_sysfs: check errors from nr_schemes file reads

selftests/damon/_damon_sysfs: check errors from nr_schemes file reads

DAMON context staging method in _damon_sysfs.py is not checking the
returned error from nr_schemes file read.  Check it.

Link: https://lkml.kernel.org/r/20240503180318.72798-3-sj@kernel.org
Fixes: f5f0e5a2bef9 ("selftests/damon/_damon_sysfs: implement kdamonds start function")
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>