export-orig crashes if the development release name isn't known

[Robie Basak]

Traceback (most recent call last):
  File "/snap/git-ubuntu/1194/usr/bin/git-ubuntu", line 11, in <module>
    load_entry_point('gitubuntu==1.0', 'console_scripts', 'git-ubuntu')()
  File "/snap/git-ubuntu/1194/usr/lib/python3/dist-packages/gitubuntu/__main__.py", line 270, in main
    sys.exit(args.func(args))
  File "/snap/git-ubuntu/1194/usr/lib/python3/dist-packages/gitubuntu/exportorig.py", line 115, in cli_main
    tarballs = main(repo, args.commit, orig_search_list)
  File "/snap/git-ubuntu/1194/usr/lib/python3/dist-packages/gitubuntu/exportorig.py", line 84, in main
    return gitubuntu.build.fetch_orig(
  File "/snap/git-ubuntu/1194/usr/lib/python3/dist-packages/gitubuntu/build.py", line 501, in fetch_orig
    tarballs = entry.mechanism(changelog, entry.source)
  File "/snap/git-ubuntu/1194/usr/lib/python3/dist-packages/gitubuntu/build.py", line 366, in fetch_orig_from_launchpad
    derive_source_from_changelog(changelog),
  File "/snap/git-ubuntu/1194/usr/lib/python3/dist-packages/gitubuntu/build.py", line 452, in derive_source_from_changelog
    return derive_source_from_series(series)
  File "/snap/git-ubuntu/1194/usr/lib/python3/dist-packages/gitubuntu/source_information.py", line 94, in derive_source_from_series
    raise ValueError("Unable to determine distribution from %s" % series)
ValueError: Unable to determine distribution from mantic