Clarify error message when you try to completely exit a pool (backport #5165)

[mergify]

This is an automatic backport of pull request #5165 done by Mergify.
Cherry-pick of c087264 has failed:

On branch mergify/bp/v15.x/pr-5165
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You are currently cherry-picking commit c0872643.
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Unmerged paths:
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	both modified:   CHANGELOG.md
	both modified:   x/gamm/pool-models/internal/cfmm_common/lp.go

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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[github-actions]

This pull request has been automatically marked as stale because it has not had any recent activity. It will be closed if no further activity occurs. Thank you!