Actionable error message for duplicate priorities in update-flag

> One small nit left IMO: I'd make the error message actionable so users understand why we can't accept ambiguous priorities. Something among the lines of `The overrides for seg1 and seg2 are in conflict; provide explicit priority values.`

_Originally posted by @khvn26 in https://github.com/Flagsmith/flagsmith/pull/8102#discussion_r3787215841_

Locate the update-flag handling for duplicate priorities and inspect the current error path. Change the message so it explains the conflict and tells users to provide explicit priority values, then confirm that duplicate priorities still produce an actionable error.

Written by the indexing model from the issue text.