More verbose error messages in `oneOf`

[kubukoz]

I like the simplicity of the current error model (just a string is sometimes exactly what you want), but currently if a union fails to decode, you get virtually no information.

Combining the errors somehow would be great to have. Some ideas:

• Either -> EitherNel (probably the most intrusive change)
• ReadError as an ADT
• just combining the strings to a larger string in the decoding logic for alternatives

[SystemFw]

I actually have some notes on errors in general, the design space is not so trivial. I'm gonna share when I get some time

[kubukoz]

Agreed, it's definitely not trivial 😅 especially if we consider error accumulation for records too.

[semenodm]

also toString for case class ReadError/WriteError is not showing actual message since both these classes are case classes and inherited from Throwable

[kubukoz]

yeah, just hit this again - @SystemFw do you have bandwidth to give errors a closer look?

For @semenodm's toString issue we can probably to a quick workaround by adding a toString override.

[SystemFw]

I'd take a PR to fix the immediate problem with toString, I don't know how long it'll be before I can look at the design more broadly

[filosganga]

I believe the #29 has addressed the issue partially: the resulting error now contains all the underlying branches failures