Disable usage of "%s"

Users can already use parameters with "$N".  This makes it so that users
do not need to escape "%" with "%%" when using named queries.

See dbcli/pgcli#865.

pgspecial/iocommands.py

@@ -188,7 +188,13 @@ def execute_named_query(cur, pattern, **_):
             query, params = subst_favorite_query_args(query, params)
             if query is None:
                 raise Exception("Bad arguments\n" + params)
-        cur.execute(query, params)
+        cur.execute(query)
+    except psycopg2.ProgrammingError as e:
+        if e.pgcode == psycopg2.errorcodes.SYNTAX_ERROR and "%s" in query:
+            raise Exception('Bad arguments: '
+                'please use "$1", "$2", etc. for named queries instead of "%s"')
+        else:
+            raise
     except (IndexError, TypeError):
         raise Exception("Bad arguments")