vcs: don't crash if submodule checkout failed
vcs: don't crash if submodule checkout failed

https://bugzilla.gnome.org/show_bug.cgi?id=728366

chergert/gnome-continuous,chergert/gnome-continuous,vrutkovs/vrutkovs.github.io,vrutkovs/vrutkovs.github.io,vrutkovs/vrutkovs.github.io,chergert/gnome-continuous