Add notification, if an api-key is missing.

src/extension.js

@@ -362,6 +362,10 @@ const OpenweatherMenuButton = new Lang.Class({
         this.refreshWeatherForecast = OpenweathermapOrg.refreshWeatherForecast;
 
         this.weatherProvider = "https://openweathermap.org/";
+
+        if (this._appid.toString().trim() == '')
+            Main.notify("Openweather", _("Openweathermap.org does not work without an api-key.\nPlease register at http://openweathermap.org/appid and paste your personal key into the preferences dialog."));
+
     },
 
     useForecastIo: function() {
@@ -407,6 +411,9 @@ const OpenweatherMenuButton = new Lang.Class({
             if (fc_locales.indexOf(locale) != -1)
                 this.fc_locale = locale;
         }
+
+        if (this._appid_fc.toString().trim() !== '')
+            Main.notify("Openweather", _("Forecast.io does not work without an api-key.\nPlease register at https://developer.forecast.io/register and paste your personal key into the preferences dialog."));
     },
 
     getWeatherProviderURL: function() {