Improve error message if `working-directory` does not exist

For justfiles that override `working-directory` value, if the dir does not exist, just gives a rather criptic message:

```just
set working-directory := 'missing'

[no-cd]
bar:
pwd # this one will work fine

foo:
pwd # this one will not even start if 'missing' dir is missing
```

```
error: Recipe ... could not be run because just could not find
the shell: No such file or directory (os error 2)
```