User-level error reporting is lacking

[fedorov]

When I get a 404 error, there is nothing that gets communicated to the user in the interface - only in the JS console. I think this should be improved. What do you think @hackermd @Punzo ?

[hackermd]

Yes, that doesn't seem to be handled correctly and should be improved.

[fedorov]

This might have been addressed in https://github.com/ImagingDataCommons/slim/pull/134 - we need to confirm, in particular by testing against various problematic situations, such as 404.