Rename ErrCampaignNoSourcesSelected to ErrCampaignMissingSources

The failure is about unconfigured campaign sources, not an empty UI
selection.

Signed-off-by: Cursor Agent <cursoragent@cursor.com>

Co-authored-by: Bryan FRIMIN <bryan@frimin.fr>

pkg/accessreview/errors.go

@@ -23,6 +23,6 @@ package accessreview
 import "errors"
 
 var (
-	ErrCampaignNoSourcesSelected = errors.New("cannot start campaign: no scope sources configured")
+	ErrCampaignMissingSources = errors.New("cannot start campaign: no scope sources configured")
 	ErrCampaignNotDraft          = errors.New("campaign must be in draft status")
 )