Don't catch Throwable

### Affected version

head

### Bug description

We do this

```
} catch (Throwable t) {
onExit(-1, t.getClass().getSimpleName());
}
```

in at least one place in GitPackCommand and in HgScmProvider. see https://stackoverflow.com/questions/6083248/is-it-a-bad-practice-to-catch-throwable for reasons not to do this

Start by locating the Throwable catch sites in GitPackCommand and HgScmProvider, then read the surrounding exit handling. Confirm the intended behavior for errors that are not ordinary exceptions and verify that both named locations no longer catch Throwable without introducing regressions.

Written by the indexing model from the issue text.