muxdb: return error when accessing the pruned revision

[libotony]

Description

Detail in https://github.com/vechain/protocol-board-repo/issues/999

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• New and existing E2E tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[otherview]

Looks good, I would merge this as the last commit and run the dup traffic tests, all clearing we should be good to go