[#72] Backend-agnostic `CofferError` GADT

[DK318]

Description

Problem

There are 2 problems here:

• In the Backend.Vault.Kv module, we're constructing OtherError with user-facing error messages as strings.
• CofferError is meant to be backend-agnostic, but it assumes that all backends will (possibly) throw a ServantError. However, the pass backend, for example, does not use servant.

Solution

1. Used ADTs + Buildable to model errors.
2. Made CofferError backend-agnostic.
3. Created backend-specific error type class.

Related issue(s)

• Fixes #72.

Fixed #

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Related changes (conditional)

• Tests

  • [ ] If I added new functionality, I added tests covering it.
  • [ ] If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • [x] I checked whether I should update the docs and did so if necessary:
  • README
  • Haddock

Stylistic guide (mandatory)

• [x] My commits comply with the policy used in Serokell.
• [x] My code complies with the style guide.

[MagicRB]

Id move VaultError into the backend folder, but aside from that LGTM

[dcastro]

Id move VaultError into the backend folder, but aside from that LGTM

Agreed. More specifically, the Backend/Vault folder.

Either to its own file or to the Backend.Vault.Kv module, whichever you prefer.