Adding 409 retry on bucket delete in BigQuery system test.

[dhermes]

As with #2307, I took down a bunch of failures last week and can't track down where this occurred, sorry.

[dhermes]

I'm not sure if we should check more than 409, there isn't much in the error:

>>> print(json.dumps(exc._errors, indent=2))
[
  {
    "domain": "global",
    "message": "The bucket you tried to delete was not empty.",
    "reason": "conflict"
  }
]
>>> exc.message
u'The bucket you tried to delete was not empty. (DELETE https://www.googleapis.com/storage/v1/b/{BUCKET})'

[daspecster]

409 could in theory...have many different messages attached to it.

Could we retry but also log/print the message incase it changes?

[daspecster]

Otherwise LGTM if you don't think we should log it.

[dhermes]

Could we retry but also log/print the message incase it changes?

Sure, but we wouldn't see it on CI, unless it failed.

I've been meaning to make the GoogleCloudError representation more verbose so we get more info.

[tseaver]

LGTM