Unclear error message for no open images, when loading model

A confusing message pops-up when attempting to load a bioimage.io model without open images.
"A RandomAccessibleInterval is required but none exist."


Happens both through ```File > Import > bioimage.io.zip``` and ```Plugins > bioimage.io > bioimage.io prediction```

Reproduce the problem through File > Import > bioimage.io.zip and Plugins > bioimage.io > bioimage.io prediction with no images open. Trace the handling of the missing RandomAccessibleInterval and replace the confusing message with a clear explanation of the required prerequisite, then verify both entry points.

Written by the indexing model from the issue text.