Should retry automatically for some kinds of errors from the API

[dobesv]

When making requests to Google APIs, you can get some HTTP status codes or network errors that indicate the request should be retried. This project doesn't offer a simple way to implement this, leaving it up to the library user to roll their own each time. It would be a great benefit to users of the library if this was handled for them automatically

HTTP status 429, 500, or 503 should be retried auotmatically with exponential back-off.

See also:

• https://developers.google.com/gmail/api/guides/handle-errors
• googleapis/google-cloud-node#460