chore: replace error string in transfer acks with const (backport #818)

[mergify]

This is an automatic backport of pull request #818 done by Mergify.
Cherry-pick of ac46ac0 has failed:

On branch mergify/bp/release/v2.1.x/pr-818
Your branch is up to date with 'origin/release/v2.1.x'.

You are currently cherry-picking commit ac46ac0.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   CHANGELOG.md
	new file:   modules/apps/transfer/types/ack.go
	new file:   modules/apps/transfer/types/ack_test.go
	modified:   modules/core/04-channel/types/acknowledgement.go

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   modules/apps/27-interchain-accounts/host/types/ack.go
	deleted by us:   modules/apps/transfer/ibc_module.go

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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[AdityaSripal]

Why is this file in the diffs?

[colin-axner]

merge conflicts most likely