dynamolock: not wrap LockNotGrantedError in error.errorString

This eases the error handling in the callers code. The old implementation had in information loss. It was not easy to check if the error was simple cased by someone else grapping the lock.

Author: hikhvar

client.go

@@ -538,8 +538,7 @@ func (c *Client) putLockItemAndStartSessionMonitor(
 
 	_, err := c.dynamoDB.PutItem(putItemRequest)
 	if err != nil {
-		err := parseDynamoDBError(err, "lock already acquired by other client")
-		return nil, fmt.Errorf("cannot store lock item: %s", err)
+		return nil, parseDynamoDBError(err, "cannot store lock item: lock already acquired by other client")
 	}
 
 	lockItem := &Lock{