storage: retry on temporary errors

Fixes #1061.

Change-Id: Ia3e8071fe59949fa003bc287fbef4d5e611fb093
Reviewed-on: https://code-review.googlesource.com/30370
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jean de Klerk <deklerk@google.com>

storage/go110.go

@@ -24,6 +24,8 @@ func shouldRetry(err error) bool {
 		// Retry on 429 and 5xx, according to
 		// https://cloud.google.com/storage/docs/exponential-backoff.
 		return e.Code == 429 || (e.Code >= 500 && e.Code < 600)
+	case interface{ Temporary() bool }:
+		return e.Temporary()
 	default:
 		return false
 	}

storage/not_go110.go

@@ -34,6 +34,8 @@ func shouldRetry(err error) bool {
 		// Unfortunately the error type is unexported, so we resort to string
 		// matching.
 		return strings.Contains(e.Error(), "REFUSED_STREAM")
+	case interface{ Temporary() bool }:
+		return e.Temporary()
 	default:
 		return false
 	}