Just reports incorrectly if the specified working directory can't be found

Say we have this `justfile`:
```justfile
[working-directory: "this-directory-does-not-exist"]
pwd:
pwd
```
`just pwd` prints
```
pwd
error: Recipe `pwd` could not be run because just could not find the shell: No such file or directory (os error 2)
```
This error message makes one think the cause is that `just` somehow cannot find the shell executable, which is not the case. The error message should point out that the problem is that instead of the shell, it's the specified working directory that cannot be found.

Reproduce the issue with the provided justfile and `just pwd`, then trace the error path for the configured working directory. Done means the failure reports that the specified working directory was not found rather than implying the shell executable is missing.

Written by the indexing model from the issue text.