More graceful handling of unsupported DICOM modalities is needed

[fedorov]

If a study contains a Structured Report instance, it will manifest itself in the error shown in #491. It would be a lot more helpful if instead it was communicated to the user that SR modality is not supported.

Also, it is not clear if after that error the viewer is expected to be usable. After dismissing the error notification, I tried to load the CT series, but that failed. If I reload the page, I will get the same error (since the SR instance in the bucket is still there), so this becomes a blocker.

[floryst]

Semi-duplicate of #486