Unhelpful error when importing a project with submodules

If we attempt to import a project that uses submodules, the error emitted is not very helpful:

Ideally, we would make the project as invalid when this happens, explaining that bzr-git does not support submodules at this time.

2009-07-16 09:46:14 INFO 65 bytes transferred |
2009-07-16 09:46:16 INFO git: Counting objects: 3 0
2009-07-16 09:46:32 INFO finding revisions to fetch:generating index 0/8147
2009-07-16 09:46:36 INFO finding revisions to fetch:generating index 0/8147
2009-07-16 09:46:39 INFO finding revisions to fetch 1
2009-07-16 09:47:39 INFO fetching revisions 385/1171
2009-07-16 09:48:39 INFO fetching revisions 679/1171
2009-07-16 09:49:39 INFO fetching revisions 936/1171
2009-07-16 09:49:55 INFO
Traceback (most recent call last):
  File "/srv/importd.launchpad.net/production/launchpad-rev-8208/scripts/code-import-worker.py", line 59, in ?
    script.main()
  File "/srv/importd.launchpad.net/production/launchpad-rev-8208/scripts/code-import-worker.py", line 54, in main
    import_worker.run()
  File "/srv/importd.launchpad.net/production/launchpad-rev-8208/lib/lp/codehosting/codeimport/worker.py", line 395, in run
    self._doImport()
  File "/srv/importd.launchpad.net/production/launchpad-rev-8208/lib/lp/codehosting/codeimport/worker.py", line 493, in _doImport
    overwrite=True)
  File "<string>", line 4, in pull_write_locked
  File "/srv/importd.launchpad.net/production/launchpad-rev-8208/lib/bzrlib/branch.py", line 855, in pull
    possible_transports=possible_transports, *args, **kwargs)
  File "/srv/importd.launchpad.net/production/launchpad-rev-8208/optionalbzrplugins/git/branch.py", line 393, in pull
    graph=graph)
  File "/srv/importd.launchpad.net/production/launchpad-rev-8208/optionalbzrplugins/git/branch.py", line 356, in update_revisions
    interrepo.fetch_objects(determine_wants, self.source.mapping)
  File "/srv/importd.launchpad.net/production/launchpad-rev-8208/optionalbzrplugins/git/fetch.py", line 437, in fetch_objects
    store, recorded_wants, pb)
  File "/srv/importd.launchpad.net/production/launchpad-rev-8208/optionalbzrplugins/git/fetch.py", line 335, in import_git_objects
    parent_invs, target_git_object_retriever._idmap, lookup_object)
  File "/srv/importd.launchpad.net/production/launchpad-rev-8208/optionalbzrplugins/git/fetch.py", line 246, in import_git_tree
    file_id, revision_id, parent_invs, shagitmap, lookup_object)
  File "/srv/importd.launchpad.net/production/launchpad-rev-8208/optionalbzrplugins/git/fetch.py", line 174, in import_git_submodule
    raise NotImplementedError(import_git_submodule)
NotImplementedError: <function import_git_submodule at 0x90040d4>
Import failed:
Traceback (most recent call last):
Failure: twisted.internet.error.ProcessTerminated: A process has ended with a probable error condition: process ended with exit code 1.