More descriptive error in GeoJson on GeoDataFrame with missing geometries (#1039)

Have GeoJson raise a more descriptive error when passed a GeoDataFrame with missing geometries.

Author: wtadler

folium/features.py

@@ -444,7 +444,7 @@ def __init__(self, data, style_function=None, name=None,
                 data = data.to_crs(epsg='4326')
             self.data = json.loads(json.dumps(data.__geo_interface__))  # noqa
         else:
-            raise ValueError('Unhandled object {!r}.'.format(data))
+            raise ValueError('Cannot render objects with any missing geometries. {!r}'.format(data))
 
         self.style_function = style_function or (lambda x: {})