gh-102541: Fix Helper.help("mod") for non-existent mod (#105934)

If the output arg to Helper() is a stream rather than the default None, which means 'page to stdout', the ImportError from pydoc.resolve is currently not caught in pydoc.doc. The same error is caught when output is None.
---------

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>