Support exception hierarchies

[bendk]

In the logins component, we currently have code like this and this. We create one error Enum, then wrap that as a variant of a top-level error Enum.

I think the most idomatic way to translate this into most languages would be an exception hierarchy like this:

• LoginsError
  • InvalidLogin
    • EmptyOrigin
    • EmptyPassword
    • ...

It would be great if UniFFI could support this. I think the main thing to figure out is how to specify it in the UDL.

┆Issue is synchronized with this Jira Task ┆Issue Number: UNIFFI-101