subrepo: do not traceback on .hgsubstate parsing errors
Note that aborting in subrepo.state() prevents "repairing" commands like revert
to be issued. The user will have to edit the .hgsubstate manually (but he
probably had already otherwise this would not be failing). The same behaviour
already happens with invalid .hgsub entries.
Ancestors of 5
4 2 0
Ancestors of 6 and 5
3 4 2 1 0
Ancestors of 5 and 4
4 2 0
Descendants of 5
7 8
Descendants of 5 and 3
6 7 8
Descendants of 5 and 4
5 7 8