Only print the errno string in case sysctl(3) does not file with ENOENT
This reduces the noise in error reporing from sysctl(8):

Before:
$ sysctl bla=something
sysctl: unknown oid 'bla': No such file or directory

After:
$ sysctl bla=something
sysctl: unknown oid 'bla'

MFC after: 1 week
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