Absent index file and parse errors handling

[kirillt]

Using ark-cli:

1. Create or copy some test folder with data.
2. Make sure there are no .ark/index file.
3. Run ark-cli monitor, wait till the index is computed, Ctrl+C.
4. Check .ark.index.

Expected: the index file exists. Actual: it is absent.

Example:

[kirill@lenovo test]$ rm .ark/index 
[kirill@lenovo test]$ ../ark-cli monitor
Building index of folder /tmp/test
Build succeeded in 12.901021531s

Updating succeeded in 287.573914ms

Updating succeeded in 283.567582ms

Updating succeeded in 292.411364ms

^C
[kirill@lenovo test]$ ls -lah .ark/index
ls: cannot access '.ark/index': No such file or directory

If you enable debug log, you can see there is an IO error in arklib (absent file).

[kirill@lenovo test]$ RUST_LOG=debug ../ark-cli monitor
Building index of folder /tmp/test
[2024-01-03T15:29:07Z INFO  arklib] Index has not been registered before
[2024-01-03T15:29:07Z INFO  arklib::index] Loading the index from file /tmp/test/.ark/index
[2024-01-03T15:29:07Z WARN  arklib::index] IO error
[2024-01-03T15:29:07Z INFO  arklib::index] Building the index from scratch
[2024-01-03T15:29:07Z DEBUG arklib::index] Discovering all files under path /tmp/test
^C

Apparently, IO errors are not handled in a nice way.