Fails if /services is not in path name

We should probably add a check for rare cases when the user omits the `/services` portion of the path name, e.g. http://sampleserver6.arcgisonline.com/arcgis/rest. ArcGIS Server treats this URL as the same as http://sampleserver6.arcgisonline.com/arcgis/rest/services, but ags-walk fails on the former.