feat(internal): provide wrapping for retried errors

[tritone]

This modifies the retry code to allow both context and service errors to be available when the call was retried until hitting a context deadline or cancelation.

This preserves the error string as it is currently put together by internal/annotate; for example a googleapi error from storage looks like this:

object.Attrs: retry failed with context deadline exceeded; last error: googleapi: got HTTP response code 503 with body: {"code":"503","message":{"error":{"message":"Retry Test: Caused a 503"}}}

Go 1.13 error semantics can be used to introspect the underlying context and service errors; here are some examples:

_, err = client.Bucket(bucketName).Object(objName).Attrs(timeoutCtx)
if err != nil {
	if e, ok := err.(interface{ Unwrap() error }); ok {
		wrappedErr := e.Unwrap()  // yields googleapi.Error type.
	}

	// errors.As allows unwrapping with access to googleapi.Error fields.
	var errAsGoogleapi *googleapi.Error
	if errors.As(err, &errAsGoogleapi) {
		log.Printf("error code: %v", errAsGoogleapi.Code)
	}

	isDeadlineExceeded := errors.Is(err, context.DeadlineExceeded)  // true
}

internal.Retry is used by the storage, datastore and bigquery clients, as well as integration tests for compute and pubsub. I want to make sure to check this with everyone affected.

[shollyman]

The wrapping of the real error seems like the big improvement here. I like it.

[codyoss]

LGTM!