[IndexedDB] Make it clearer that aborted upgrade transaction deletes new stores/indexes (#63)

It is pretty clear that when an __upgrade transaction__ aborts the newly created stores and indexes go bye-bye. But it's not explicit that they are considered _deleted_ for the purposes of various algorithms, e.g. clauses like _"if store has been deleted, throw an InvalidStateError exception"_

I'm not sure the best place to put this. Possibly in the first paragraph of [Steps for aborting a transaction]( https://w3c.github.io/IndexedDB/#dfn-steps-for-aborting-a-transaction)

---
Reply to this email directly or view it on GitHub:
https://github.com/w3c/IndexedDB/issues/63