m.push_rules, like m.fully_read, is a special account data type that cannot
be set using the normal /account_data endpoint. Return an error instead
of allowing data that will not be used to be stored.
A dont_notify action is a no-op (and coalesce is undefined). These are
both considered no-ops by the spec, per MSC3987 and the predefined
push rules were updated to remove dont_notify from the list of actions.
