Fail test in case of double onError/onComplete signals

While updating some Akka streams internals @patriknw discovered we accidentally called onError twice on the overflow protection. We believe `onError` should be a "final" call, according to https://github.com/reactive-streams/reactive-streams#1.7 (the situational rules mentioned in this rule don't apply here IMO).

This addition should help to notice such implementation mistakes sooner. Please review @reactive-streams/contributors (I somehow can't get this to be a "ping" hm...).

This PR changes:
* `spec317_mustSignalOnErrorWhenPendingAboveLongMaxValue` to check that no other errors than the one expected error were signalled
* in general protects from multiple `onError` and `onComplete` calls on the same subscriber. The situational rule linked from `1.7` still holds as it states "`onError` if `onComplete` failed", which is still allowed with this update of course (we only disallow multiple `onError / onComplete` calls)