Michael suggested that a bar line should be drawn in LilyPond at the place
where a volta begins.
This is now done using a very simple but crude way.
It works at least with LilyPond 2.13.
If it doesn't work with some other LilyPond revision it could be disabled with
a new checkbox in the LilyPond exporter dialog.
The way this is done is provisional. I will rewrite it in a more robust way as
soon as I will find time to cleanup the LilyPond exporter code.