Update help and handle missing resources in download.py (see #12146).

This is a follow-up PR to https://github.com/openmicroscopy/openmicroscopy/pull/2383. It fixes two small bugs discovered by @pwalczysko in https://github.com/openmicroscopy/openmicroscopy/pull/2398:
- the text of the help message in ``bin/omero download -h`` should make more sense now,
- when a resources is deleted in ``omero.data.dir`` (Image, File etc.), a ``ResourceError`` is raised by the server. We now print a nicer message instead of a full stack trace.