Catch potential custom network errors in docker

builtin/providers/docker/resource_docker_container_funcs.go

@@ -162,8 +162,11 @@ func resourceDockerContainerCreate(d *schema.ResourceData, meta interface{}) err
 	if v, ok := d.GetOk("networks"); ok {
 		connectionOpts := dc.NetworkConnectionOptions{Container: retContainer.ID}
 
-		for _, network := range v.(*schema.Set).List() {
-			client.ConnectNetwork(network.(string), connectionOpts)
+		for _, rawNetwork := range v.(*schema.Set).List() {
+			network := rawNetwork.(string)
+			if err := client.ConnectNetwork(network), connectionOpts); err != nil {
+				return fmt.Errorf("Unable to connect to network '%s': %s", network, err)
+			}
 		}
 	}