ArcGIS XYZ services choke trying to get legend

[rhodges]

see 36af808 -- should we ever get here for XYZ, even if served via an Esri MapServer?

• fail gracefully?
• Test for legend support from server?
• Avoid this altogether, assume XYZ does not have legend services?

[rhodges]

Here is the url template for service that wouldn't load w/o providing a legend URL: https://tiles.arcgis.com/tiles/C8EMgrsFcRFL6LrL/arcgis/rest/services/DSCRTP/MapServer/tile/{z}/{y}/{x}