Return warnings from service create and service update when digest pinning fails

Modify the service update and create APIs to return optional warning
messages as part of the response. Populate these messages with an
informative reason when digest resolution fails.

This is a small API change, but significantly improves the UX. The user
can now get immediate feedback when they've specified a nonexistent
image or unreachable registry.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

command/stack/deploy.go

@@ -408,15 +408,20 @@ func deployServices(
 			if sendAuth {
 				updateOpts.EncodedRegistryAuth = encodedAuth
 			}
-			if err := apiClient.ServiceUpdate(
+			response, err := apiClient.ServiceUpdate(
 				ctx,
 				service.ID,
 				service.Version,
 				serviceSpec,
 				updateOpts,
-			); err != nil {
+			)
+			if err != nil {
 				return err
 			}
+
+			for _, warning := range response.Warnings {
+				fmt.Fprintln(dockerCli.Err(), warning)
+			}
 		} else {
 			fmt.Fprintf(out, "Creating service %s\n", name)