cherry-pick of "Merge branch '21.6-Powheg_MadSpin_ErrorLog_Fix' into '21.6'"

Timothee Theveneaux-Pelzerrequested to merge
cherry-pick-85971c5d into master

PowhegControl: bugfix in MadSpin log messages which was triggering an error by mistake - AGENE-2086

See merge request !47414 (merged)

(cherry picked from commit 85971c5d)

1d84315f PowhegControl: bugfix in MadSpin log messages which was triggering an error by mistake - AGENE-2086

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