Downgrade ERROR when trying to thin non-existent forward track particles
Alternative fix for ATLASRECTS-6827 without changing output wrt previous reference.
Following the observation that !50811 (merged) caused output differences when porting the behaviour from master back into 22.0-mc20 this was looked at more carefully, and discovered that the fix in master may not be completely correct.
Given the different potential cases that need to be covered, the simplest fix seems to just be to downgrade the error to an info message, since there seem to be legitimate cases where it could get triggered.