[DSP-86] Make old code compatible with new errors

Description

Problem: after adding mandatory ErrorHasDoc instances old code which declared errors as datatypes does not work anymore.

Solution: add stub instance of ErrorHasDoc to deriveCustomError.

Old approach to errors does not support proper documentation and is not going to, so this solution seems fine.

Related issue(s)

https://issues.serokell.io/issue/DSP-86

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • 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

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

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.