Provide a custom retryablehttp.ErrorHandler for more consistent returns using retries.

[andrewsomething]

This aims to solve a number of inconsistencies/issues when using WithRetryAndBackoffs. Currentlywhen a retry happens:

• The final error response from the API is overwritten with the "giving up after x attempt(s)" error message. It does not include the final status code.
• The error gets double nested. So calling err.Error() returns the messy looking Get "http://127.0.0.1:45035/foo": GET http://127.0.0.1:45035/foo giving up after 4 attempt(s)
• The *godo.Response returned by the resource methods is nil if there is an error. That is not consistent with existing godo or net/http. If there is an HTTP response, even an errpr, it should be returned.

This uses a custom ErrorHandler in order to provide errors that are consistent with a *godo.ErrorResponse and a non-nil *godo.Response while still providing insight.

[danaelhe]

Just tried it in terraform and works well 👍 thanks!