Emit more informative error message when cwd does not exist

[smoelius]

Fixes #72 in the manner described in https://github.com/matklad/xshell/issues/72#issuecomment-1627735581.

Nits are welcome.

[matklad]

yup!

[matklad]

bors r+

[matklad]

bors retry

[bors[bot]]

Already running a review

[matklad]

bors r-

[bors[bot]]

Canceled.

[matklad]

bors r+

[smoelius]

~Want me to rebase?~ (I think I'm too late.)

[matklad]

No need to rebase, bors shold do the right thing here.

[bors[bot]]

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• publish

[matklad]

should be published as 0.2.4 if I didn't mess up my new scoped token:

https://github.com/matklad/xshell/pull/74

[smoelius]

Thanks!