Fix: Identities Group Management - bessere Fehlermeldungen

- add_user_to_group: Exception werfen mit stderr Nachricht
- remove_user_from_group: Exception werfen mit stderr Nachricht
- text=True für subprocess für besseres Error Handling
- Router aktualisiert um Fehlermeldungen an Frontend weiterzugeben
- Benutzer sehen jetzt detaillierte Fehlermeldungen beim Gruppe-Entfernen

Behebt: 'Failed to remove user from group' verschluckt die echte Fehlermeldung

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/routers/identities.py

@@ -229,12 +229,10 @@ async def add_user_to_group(
 ):
     """Add user to group"""
     try:
-        success = identities_manager.add_user_to_group(username, request.groupname)
-        if not success:
-            raise HTTPException(status_code=400, detail="Failed to add user to group")
+        identities_manager.add_user_to_group(username, request.groupname)
         return {"status": "added", "username": username, "groupname": request.groupname}
     except Exception as e:
-        raise HTTPException(status_code=500, detail=str(e))
+        raise HTTPException(status_code=400, detail=str(e))
 
 
 @router.delete("/users/{username}/groups/{groupname}")
@@ -245,12 +243,10 @@ async def remove_user_from_group(
 ):
     """Remove user from group"""
     try:
-        success = identities_manager.remove_user_from_group(username, groupname)
-        if not success:
-            raise HTTPException(status_code=400, detail="Failed to remove user from group")
+        identities_manager.remove_user_from_group(username, groupname)
         return {"status": "removed", "username": username, "groupname": groupname}
     except Exception as e:
-        raise HTTPException(status_code=500, detail=str(e))
+        raise HTTPException(status_code=400, detail=str(e))
 
 
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