Looks like the 10ft elevation contour REST service (https://services.nationalmap.gov/arcgis/rest/services/USGSTopoLarge/MapServer/35) was removed from the National Map. I made the mistake of deleting the portal layer before removing it from my map application. How can I remove the defunct layer as it does not show up in the layers list, it just gives the error every time that it can't load it.
