reorder force flush condition + add 502 as retriable error

[yan-hic]

• fixes does not handle 502 errors (returned by GCS when server is overloaded) #31
• add error code 502 as retriable error as per https://cloud.google.com/storage/docs/json_api/v1/status-codes#502_Bad_Gateway

[yan-hic]

I think the test must be adapted

[martindurant]

Yes, the reordering did this, but I think that the logic now is essentially right, but that write should check for forced=True and raise the error there. Getting the tests to work is, in general, a bit of a pain, because they rely on vcrpy, which is rather flaky.

Sorry I haven't been able to dedicate any effort to this project. I hope to come back to it and clean up the several small-but-annoying issues.

[yan-hic]

Thanks @martindurant and team for Pythonic file-object support of GCS.

Why Google has not this covered yet in their client library is beyond me.
As a fyi, the arguments for it, lack of traction and perceived resistance: googleapis/google-cloud-python#3903

In the meantime I am researching how to use service accounts for GCSFS credentials as this fits a use case of mine.

[martindurant]

Superseded by #37