wal: OpenFromIndex fails if it cannot find previous index

Example:
We save entry 1, 2, 3 to WAL.
If we try to open 100, it should fail.

#1033 point 2.
Perhaps you could disambiguate between the two cases:

```
ErrFileNotFound = errors.New("wal: file not found")
ErrIndexNotFound = errors.New("wal: index not found in file")
```