In multi-edit mode, the select non-manifold function would exit with
an error if any mesh was in face mode.
While in practice the mode is synced between meshes, it's possible for
them to get out of sync with multiple scenes.
Editing in the middle of a loop on all edit-objects would change their
selection based on the internal order, leaving some unhandled,
returning canceled even though changes where made.
Resolve by checking the selection mode in the operators poll function,
then ensure all edit-meshes selection modes match the active object.
4b6ebc9579