Fix compiling issue in Swift bindings for async throw functions

[crazytonyli]

<ErrorType>.lift Swift functions are private API and are referenced in errorHandler: {{ e|ffi_error_converter_name }}.lift. That would cause compiling issues in Swift bindings of a crate that uses external error types in async functions.

The first commit reproduces the issue, and the second commit fixes the problem.

[crazytonyli]

BTW, this PR is potentially related to #2153. I'm not entirely sure because I haven't read the entire discussion there.

[bendk]

This looks good to me, thanks for the patch!

Can you merge this together into 1 commit and re-push? I think 1 commit is better to merge, since it works better with git-bisect. Also, I'm hoping that a re-push will unbreak the circle CI pipeline.

[crazytonyli]

@bendk Done. All changes are in one commit now.

[bendk]

We're really close here:

• Can you rebase one more time? We just merged a fix for the rustup breakage. Hopefully that will make CI run.
• When I run this with Swift 6, I get errors from calling an async function outside of an async context. I updated the test to use a task here: 78cdd16 how does that look to you? Feel free to merge it or make some other changes, I'm not tied to that code.

[crazytonyli]

@bendk I have added your changes to this PR. It's probably an "issue" of Swift Linux, because I can run the test just fine without Task on my mac.

[bendk]

Still not sure what's going on with CI, but this change looks good to me.