Improve error messages for derived sources

There seemed to be confusion over reactive-scoping (likely due to
incomplete docs), so I've added more explicit error messages when
deriving in non-reactive scopes to warn the user to use `root()` or
`mount()`.

src/cleanup.luau

@@ -7,8 +7,9 @@ local add_cleanup = graph.add_cleanup
 
 local function cleanup(callback: () -> ())
     local scope = get_scope()
-    if not scope then 
-        throw("cannot cleanup in a non-reactive scope")
+
+    if not scope then
+        throw "cannot cleanup in a non-reactive scope"
     end; assert(scope)
 
     add_cleanup(scope, callback)