In the previous code `PachliError` could correctly chain errors and
generate error messages, `ApiError` didn't, which is why there was the
temporary `ApiError.fmt()` extension function.
Rewrite `ApiError` to implement `PachliError` so it gets these benefits
and to reduce the number of different error-handling mechanisms in the
code.
Main changes:
- `PachliError` is now an interface so it can be extended by other
error interfaces.
- All the `ApiError` subclasses implement `PachliError`, and can
specify the error string and interpolated variables at the point of
declaration.
- Update `ListsRepository` and `ServerRepository` to return
`PachliError` subclasses.
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