The `ErrFileNotFound` was used for for three cases:
1. There is no any WAL files (probably due to no read permission);
2. There is no WAL files matching the snapshot index;
3. The WAL file seqs do not increase continuously.
It's not good for debug when users see the `ErrFileNotFound` error,
so in this PR, a different error is returned for each case above.
Signed-off-by: Benjamin Wang <wachao@vmware.com>