See also #27675
Fix logic problem introduced by #29413, which is serializer tries to
merge statslog list while level segments do not have statslog. This
shall result returning error. `writeBufferBase` ignores this error but
it shall only ignore `ErrSegmentNotFound`.
This PR add logic checking segment level before execution of merging
statslog list. And add error type check for getSyncTask failure.
Signed-off-by: Congqi Xia <congqi.xia@zilliz.com>
congqixia