refact: removeAllUsersFromChannel to return an error

[moogacs]

Summary

refactor removeAllUsersFromChannel to feedback the error on return instead of just printing

Ticket Link

Fixes https://mattermost.atlassian.net/browse/MM-47341
Issue: mattermost/mattermost#21218

[mattermod]

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[moogacs]

/check-cla

[mattermod]

This PR has been automatically labelled "stale" because it hasn't had recent activity.
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Thank you for your contribution!

[noxer]

Thank you very much for your contribution, @moogacs. I'd like to request a few changes and there seem to be linter errors to fix (a simple go fmt should take care of those).

[noxer]

Thank you very much for your contribution @moogacs.
The PR looks good. I've added a nit but feel free to ignore it, it's not that important.

[isacikgoz]

LGTM, thanks @moogacs 🎉 good to merge after what @noxer pointed out.