[BP] MB-9692 Handle MutationLog lseek errors gracefully

Instead of asserting throw appropriate exception with logs.

Change-Id: Ib64b5bea506e2524cada118287e9e2d300b970e0
Reviewed-on: http://review.couchbase.org/32697
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>