Error correctly when trying to access a directory that doesn't exist

```
[project]
name = "cmkr_printup_bug"

[target.cmkr_printup_bug]
type = "executable"
sources = ["src/cmkr_printup_bug/*.cpp"]
compile-features = ["cxx_std_11"]
```

(presumably this errors if `src/cmkr_printup_bug` doesn't exist