don't crash with an invalid api key

controllers/weather.php

@@ -16,7 +16,7 @@ $app->get('/api/weather', function() use($app) {
     } else {
       json_response($app, [
         'error' => 'not_found',
-        'error_description' => 'No weather information was found for the requested location'
+        'error_description' => 'No weather information was found for the requested location, or you used an invalid API key'
       ]);
     }
   } else {

p3k/Weather.php

@@ -9,6 +9,9 @@ class Weather {
     $data = self::_fetch($lat, $lng, $key);
     if(!$data) return null;
 
+    if(!property_exists($data, 'current_observation'))
+      return null;
+
     $current = $data->current_observation;
 
     $weather = [