[streams] Notate abstract ops that can throw (#371)

#366 was a good first step in making the flow of potential exceptions clearer in the spec. We should also consider notating whether an abstract operation can throw or not so that callers know whether to use perform or to handle any abrupt completions.

Here's an approach:
![Sample screenshot hacked together with web inspector](https://cloud.githubusercontent.com/assets/617481/8471094/a55b5fe6-20cf-11e5-956b-11bf6d9343fc.png)


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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/371