Fix error text on failed reading of parameter.
Fix error text on failed reading of parameter.

ECGROUPVALUENOTEXIST is translated by cgroup_strerr to "Cgroup trying to set
value for control that does not exist". This is wrong, because the error
code is being returned also when *reading* non-existing value,

I propose "Cgroup, requested group parameter does not exist".

Signed-off-by: Jan Safranek <a7f4b96ffdd515ecef00f117294da18555749b0b@redhat.com>

jrfastab/Linux-cgdcbxd,jrfastab/Linux-cgdcbxd,rafalmiel/libcg,jrfastab/Linux-cgdcbxd,rafalmiel/libcg,rafalmiel/libcg,jrfastab/Linux-cgdcbxd,rafalmiel/libcg