libct: drop "from parent" from generic sync errors

These messages date back to 4776b4326 ("libcontainer: refactor syncT
handling"), which added libcontainer/sync.go. Its readSync was used by
"runc init" only, to read what its parent had sent, so saying "parent"
was accurate.

f81ef1493 ("libcontainer: sync: cleanup synchronisation code") split
readSync into doReadSync plus a few wrappers, and doReadSync now serves
both sides: the child reaches it through readSync/readSyncFull, and the
parent through parseSync. The wording was carried over unchanged, so
ever since, a read error on the parent side claims runc failed to read
from its parent, while it was in fact reading from "runc init".

Both messages are equally clear without naming a side.

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

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

libcontainer/sync.go

@@ -131,10 +131,10 @@ func doReadSync(pipe *syncSocket) (syncT, error) {
 			logrus.Debugf("sync pipe closed")
 			return sync, err
 		}
-		return sync, fmt.Errorf("reading from parent failed: %w", err)
+		return sync, fmt.Errorf("reading sync failed: %w", err)
 	}
 	if err := json.Unmarshal(packet, &sync); err != nil {
-		return sync, fmt.Errorf("unmarshal sync from parent failed: %w", err)
+		return sync, fmt.Errorf("unmarshal sync failed: %w", err)
 	}
 	logrus.Debugf("read sync %s", sync)
 	if sync.Type == procError {