examples: check return value of nvs_flash_init

nvs_flash_init may return an error code in some cases, and applications
should check this error code (or at least assert on it being ESP_OK, to
make potential issues more immediately obvious).

This change modifies all the examples which use NVS to check the error
code. Most examples get a simple ESP_ERROR_CHECK assert, while NVS
examples, OTA example, and NVS unit tests get a more verbose check which
may be used in real applications.

examples/protocols/mdns/main/mdns_example_main.c

@@ -178,7 +178,7 @@ static void mdns_task(void *pvParameters)
 
 void app_main()
 {
-    nvs_flash_init();
+    ESP_ERROR_CHECK( nvs_flash_init() );
     initialise_wifi();
     xTaskCreate(&mdns_task, "mdns_task", 2048, NULL, 5, NULL);
 }