Handle partial/failed write in the index file

[Thomasdezeeuw]

Currently if the Index file has an incorrect length it will refuse to open the file:

stored/src/storage/mod.rs

Lines 1082 to 1086 in d3604d0

	} else if (length as usize - INDEX_MAGIC.len()) % size_of::<Entry>() != 0 {
	return Err(io::Error::new(
	io::ErrorKind::InvalidData,
	"incorrect index file size",
	));

However we should handle it more cleanly, ignoring the last partial/invalid entry for example. This will require more checks on the index entries themselves.