[#385] Preserve docs after failWith

Description

Problem: it's relatively easy to accidentally add doc items after failWith, and those will be silently stripped, which is, to say the least, suboptimal.

Solution: preserve all docs after failing instructions, but strip everything not documentation-related.

Related issue(s)

Resolves #385 (closed) (partially?)

✅ 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:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

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• My code complies with the style guide.