fix: Add ABORTED to retryable status codes.

[chrisrossi]

Fixes #383.

[chrisrossi]

A little bit of background discussion, since even though it's a simple change, the implications are somewhat complicated:

Andrew Gorcester, Tue 5:22 PM

@chris Rossi Earlier I said ABORTED could be retried; I thought better of it and dug a little deeper and it's a bit more complicated than that
From Clem: "the semantics of ABORTED is: "retry at a higher level"
for transactions, this means retrying the complete transaction, not only the Commit RPC"
That suggests that some RPCs could potentially be retried if they're self-contiained but many cannot be, they would need user interaction or code that retries the entire process from the start

Chris Rossi, Yesterday 11:35 AM

Ok. I think NDB has retry for transactions and for individual RPCs. I'll take a closer look and see if I can't tease out a way to apply this to the former and not the latter.

Chris Rossi, Yesterday 4:33 PM

So, I'm poking around, and if we're in a transaction, and we get a retryable error, we already retry the entire transaction. So if we add ABORTED to our list retryable codes, we'll be following the semantics of ABORTED when we're in a transaction. If we're not in a transaction, then we shouldn't see it, would be my guess, so having it in the list shouldn't make any difference one way or the other for non-transactions. Does that seem reasonable?

Andrew Gorcester, Yesterday 4:34 PM

hmm... I think so, yeah.

[cguardia]

Looks good.