statichttprepo: use LockUnavailable() instead of Abort() for lock (API) If we try to get a lock on statichttprepo, we get `error.Abort()` instead of subclass of `error.LockError()`. The callers which catches `error.LockError` fails to catch this case as the correct error is not raised. Raising `error.LockUnavaible()` is same as what is done for wlock also.