Fix toEraInMode for conway

[newhoggy]

Description

The pattern match for ConwayEra era was missing and is added.

Also restructure the pattern match so this doesn't happen again for future eras.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. These may include:
  • golden tests
  • property tests
  • roundtrip tests
  • integration tests
    See Runnings tests for more details
• Any changes are noted in the CHANGELOG.md for affected package
• The version bounds in .cabal files are updated
• CI passes. See note on CI. The following CI checks are required:
  • Code is linted with hlint. See .github/workflows/check-hlint.yml to get the hlint version
  • Code is formatted with stylish-haskell. See .github/workflows/stylish-haskell.yml to get the stylish-haskell version
  • Code builds on Linux, MacOS and Windows for ghc-8.10.7 and ghc-9.2.7
• Self-reviewed the diff

Note on CI

If your PR is from a fork, the necessary CI jobs won't trigger automatically for security reasons.
You will need to get someone with write privileges. Please contact IOG node developers to do this
for you.

[carbolymer]

I'm wondering if it would make sense to add a property test to catch similar bugs in the future.

[newhoggy]

The type system should be able to catch this. By removing _, not adding new eras in the pattern match will make it in-exhaustive so we'll get an error.

[carbolymer]

Assuming that the mapping is correct in the first place 😃

[lehins]

Yeah, avoiding catch all _ in this kind of functions is a really good practice!

👍