gh-ost migrations: improved error log message

[shlomi-noach]

Description

This PR merely changes the text in an error message when the OnlineDDL executor finds and terminates a rogue gh-ost migration.

Related Issue(s)

#6926

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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