fix(esp_modem): Update ap2ppp example to recover network on disconnection

Closes https://github.com/espressif/esp-protocols/issues/44

components/esp_modem/test/target/main/NetworkDCE.cpp

@@ -87,7 +87,7 @@ esp_err_t modem_init_network(esp_netif_t *netif)
     return NetModule::init(netif);
 }
 
-void modem_start_network()
+esp_err_t modem_start_network()
 {
     NetModule::start();
 }