Enable need-check-nil and start the burn down

`Roles.new_role` returned the result of `error`, which made it nullable,
and `set_permission_group` returned nil for an unknown group, which
broke the config chain with an index error rather than saying so.

Positions and bounding boxes read back from the game always use the
named members, so they are narrowed once where they enter a function.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

exp_legacy/module/expcore/roles.lua

@@ -719,7 +719,7 @@ local role = Roles.new_role('Moderator', 'Mod')
 ]]
 function Roles.new_role(name, short_hand)
     ExpUtil.assert_not_runtime()
-    if Roles.config.roles[name] then return error("Role name is non unique") end
+    if Roles.config.roles[name] then error("Role name is non unique") end
     local role = setmetatable({
         name = name,
         short_hand = short_hand or name,
@@ -896,8 +896,7 @@ function Roles._prototype:set_permission_group(name, use_factorio_api)
     if use_factorio_api then
         self.permission_group = { true, name } --[[@as [boolean, string] ]]
     else
-        local group = Groups.get_group_by_name(name)
-        if not group then return end
+        assert(Groups.get_group_by_name(name), "Permission group not found: " .. name)
         self.permission_group = name
     end
     return self