[#748] Move `senderIsNotAdmin` Lorenz error to `morley-ledgers` repo

Description

Move senderIsNotAdmin error over to morley-ledgers repo; see also companion MRs: indigo!30 (closed), morley-ledgers!58 (merged)

Related issue(s)

Partly resolves #748 (closed).

✅ 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.

Stylistic guide (mandatory)

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