Choropleth: add message when key_on not found in data (#1144)

Raise a value error when the `key_on` parameter was not found in the geojson data in Choropleth.

Author: evwhiz

folium/features.py

@@ -1109,6 +1109,8 @@ def get_by_key(obj, key):
 
             def color_scale_fun(x):
                 key_of_x = get_by_key(x, key_on)
+                if not key_of_x:
+                    raise ValueError("key_on `{!r}` not found in GeoJSON.".format(key_on))
 
                 if key_of_x not in color_data.keys():
                     return nan_fill_color, nan_fill_opacity