Add progress indicator. Handle errors.
Add progress indicator. Handle errors.

Issue:
https://github.com/genenetwork/gn-gemtext-threads/blob/main/topics/gn1-migration-to-gn2/clustering.gmi

* Add a progress indicator to show the user that there is some
  progress happening, so that they can wait.

  Add a function to handle any errors that arise.

pjotrp/genenetwork2,pjotrp/genenetwork2,pjotrp/genenetwork2,pjotrp/genenetwork2,genenetwork/genenetwork2,genenetwork/genenetwork2,genenetwork/genenetwork2,genenetwork/genenetwork2,pjotrp/genenetwork2