Improve error message when loading RTSTRUCT from manfest.json URL

[PaulHax]

Hard to understand error message when trying to load a DICOM VolView does not support via a manifest.json file via URL parmeter

https://volview-dev.kitware.app/?urls=[https://data.kitware.com/api/v1/file/686549184263e86bc81ba338/download/manifest.json]

Just getting the file directly not through manifest.json makes a little more sense:

https://volview-dev.kitware.app/?urls=[https://data.kitware.com/api/v1/file/686544a24263e86bc81ba335/download/1-1-rtstruct-lung.dcm]