Various cleanup / minor issues

* 7.1 Error codes needs some "MUST" https://github.com/adlnet/xAPI-Spec/issues/97
    * Tightened the language to "most appropriate" (400 would be appropriate anywhere 403 would be, but 403 should be used when it's "most appropriate").
    * Removed language which implied 400 return codes were optional, or that other return codes could not appear on any resource

* stored shouldn't be in the future, remaining issue was actually the sub-statement reference (which I removed) --  https://github.com/adlnet/xAPI-Spec/issues/105
* stored/timestamp precision https://github.com/adlnet/xAPI-Spec/issues/83