Check for a source property that's actually used

The existence of a features property was checked for, but a different property was passed to create an ol.source.GeoJSON or ol.source.TopoJSON. This updates the conditions to check for the source.geojson and source.topojson properties.
Update error messages to reflect correct property
Merge pull request #11 from benverhees/master

Fix error when using a GeoJSON object instead of a url for a source