libct/cgroups/readProcsFile: ret errorr if scan failed

Not sure why but the errors from scanner were ignored. Such errors
can happen if open(2) has succeeded but the subsequent read(2) fails.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/utils.go

@@ -138,7 +138,7 @@ func readProcsFile(file string) ([]int, error) {
 			out = append(out, pid)
 		}
 	}
-	return out, nil
+	return out, s.Err()
 }
 
 // ParseCgroupFile parses the given cgroup file, typically /proc/self/cgroup