[#183] Improve error handling in morley-client, part 2

Description

This MR makes errors thrown by morley-client more convenient to deal with. Note that there are still a lot of cases that need special handling, but I have already spent too much time on this issue and I have many other things to do. So I have created a follow-up issue: #284.

Related issue(s)

Resolves #183 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it ← part of this MR is just refactoring that doesn't need tests. Some logic should be tested as part of cleveland as it evolves I think.
  • 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.

Stylistic guide (mandatory)

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