Row level security returning no error when blocking all effects of a query
If you only enable RLS for a table, but do not specify rules, then a "deny all" rule is implicitly assumed.

Quote from the manual


  When row security is enabled on a table [...] all normal access to the table for selecting rows or modifying rows must be allowed by a row security policy. [...] If no policy exists for the table, a default-deny policy is used, meaning that no rows are visible or can be modified.


(emphasis mine)

So since the policy makes no rows visible, this is why there is no error when UPDATEing and you get 0 rows affected.