Flatten rcl error type

[nnmm]

As discussed in chat:

• The flatter error type is easier to match against
• The flat model is less code and less noise in the package docs from the many types.
• The "sub-error types" we had before were not actually useful in most cases (e.g. I can't imagine someone wanting to take some specific action in response to all four NodeError variants).
• Other client libraries do not have a hierarchy for these error codes either.

I removed the tests because they didn't seem terribly useful, but we can discuss this.

[esteve]

Given that TryFrom is implemented, TryInto should already be available for free (https://doc.rust-lang.org/std/convert/trait.TryInto.html).

[nnmm]

Yes, but isn't using to_rclrs_result nicer here, since this function fetches the error message from rcutils?

Using try_into()/try_from() also makes this call site more complicated. It would have to be something like this:

        match RclReturnCode::try_from(rc) {
            Err(e) => panic!("Unable to release WaitSet. Unknown error code {:?}", e),
            Ok(e) if e != RclReturnCode::Ok => panic!("Unable to release WaitSet. {:?}", e),
        }

[esteve]

This looks much nicer and more idiomatic as it leverages TryInto:

if let Err(e) = TryInto::<RclReturnCode>::try_into(rc) {
    panic!("Unable to release WaitSet. {:?}", e)
}

And then we can get rid of to_rclrs_result entirely and incorporate it into TryFrom

[nnmm]

This does not do what you mean. TryInto is just Err if the error code is unknown.

[esteve]

I see what you mean. It somehow feels like a code smell that we have this function outside all the other conversion traits and only used in one specific place. If we can find a way to refactor this in the future, I'd be much happier. For now that's ok.

[nnmm]

Yes, I also hope we can solve this more elegantly. Thanks for reviewing!

[nnmm]

@esteve Could you also take another look at this?

[esteve]

This looks much nicer and more idiomatic as it leverages TryInto:

if let Err(e) = TryInto::<RclReturnCode>::try_into(rc) {
    panic!("Unable to release WaitSet. {:?}", e)
}

And then we can get rid of to_rclrs_result entirely and incorporate it into TryFrom