Overrides the path to the `oopsie` crate used in the generated code. Needed when
`oopsie` is re-exported under a different path or renamed in `Cargo.toml`.
Defaults to `::oopsie`.
Forms: `path = "some::path"`, `path = some::path`.
```ignore
// A real custom path must name a crate that actually re-exports `oopsie`,
// which can't exist inside this crate's own doctest.
pub enum AppError {
}
```