Use flex-error to define errors

[soareschen]

Description

This PR experiments on using flex-error to define application errors using the define_error! macro. Using this, we can easily switch between error tracers such as eyre, anyhow, or a naive string tracer by triggering Cargo feature flags.

For the experiment, only the error definition ics26_routing::error is modified. To inspect how the error definitions are expanded, install cargo-expand and run the following:

$ cd modules && cargo expand ics26_routing::error

More documentation will be written on the flex-error GitHub.

---

For contributor use:

• Updated the Unreleased section of CHANGELOG.md with the issue.
• If applicable: Unit tests written, added test to CI.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments.
• Re-reviewed Files changed in the Github PR explorer.

[soareschen]

The documentation for flex-error is now available at https://docs.rs/flex-error/0.2.0/flex_error/

[soareschen]

A new version of flex-error is now published: https://docs.rs/flex-error/0.4.0/flex_error/

Hopefully the documentation is now clear enough that future contributors can understand how to define and update the error types here.

[romac]

Great work @soareschen! I have one question left: I see that by default flex-error pulls in both anyhow and eyre, and I wonder if we even need to pull those in? If we need one of the two library, then we can perhaps only enable eyre?

[romac]

Can we also add an entry to .changelog? See #1232 and https://github.com/informalsystems/unclog for some background.

[soareschen]

I have one question left: I see that by default flex-error pulls in both anyhow and eyre, and I wonder if we even need to pull those in? If we need one of the two library, then we can perhaps only enable eyre?

I have updated the the dependency to flex-error with default-features = false, and enable the flex-error/eyre_tracer feature by default. So now the default ibc crate will only have eyre as the indirect dependency. The user of ibc crate should be able to override this to use anyhow if they want.

In any case, I leave both options for eyre and anyhow in flex-error because it was undecided which error tracer would work better. With the option there, we can still switch from eyre to anyhow easily if there are issues with eyre in the future.