improved shared stops error messages

[miles-grant-ibigroup]

Checklist

• [x] Appropriate branch selected (all PRs must first be merged to dev before they can be merged to master)
• [x] Any modified or new methods or classes have helpful JavaDoc and code is thoroughly commented
• [x] The description lists all applicable issues this PR seeks to resolve
• [x] The description lists any configuration setting(s) that differ from the default settings
• [x] All tests and CI builds passing
• [ ] The description lists all relevant PRs included in this release (remove this if not merging to master)
• [ ] Code coverage does not significantly worsen (ideally it improves) (remove this if not merging to master)

Description

Some of the shared stops error messages were confusing! Or didn't reflect reality. This PR resolves this issue.

[miles-grant-ibigroup]

Wrong repo, apologies.