Here is example of present behavior with the -n option to specify an alternate repo:

./download.py -n craigdo/ns-3-names

    #
    # Get NS-3
    #
    
Cloning ns-3 branch
 =>  hg clone http://code.nsnam.org/craigdo/ns-3-names ns-3-names
requesting all changes
adding changesets
adding manifests
adding file changes
added 4119 changesets with 14765 changes to 1633 files
797 files updated, 0 files merged, 0 files removed, 0 files unresolved

    #
    # Get the regression traces
    #
    
Synchronizing reference traces using Mercurial.
 =>  hg clone http://code.nsnam.org/ns-3-names-ref-traces ns-3-names-ref-traces
abort: HTTP Error 404: Not Found
Traceback (most recent call last):
  File "./download.py", line 184, in <module>
    sys.exit(main())
  File "./download.py", line 177, in main
    get_regression_traces(ns3_dir)
  File "./download.py", line 47, in get_regression_traces
    run_command(["hg", "clone", constants.REGRESSION_TRACES_REPO + regression_traces_dir_name, regression_traces_dir_name])
  File "/home/tomh/hg/ns-3-allinone/util.py", line 24, in run_command
    raise CommandError("Command %r exited with code %i" % (argv, retval))
util.CommandError: Command ['hg', 'clone', 'http://code.nsnam.org/ns-3-names-ref-traces', 'ns-3-names-ref-traces'] exited with code 255

-------------


I suggest that if craigdo/ns-3-names-ref-traces is not found in this case, the script prints out a warning such as "No matching reference traces found for craigdo/ns-3-names-ref-traces" and moves on.