This string had an incorrect syntax (should have been '_("foo") % e'),
which made mainwindow's save-project-failed handler choke in some cases
(such as overwriting a project that has been marked read-only).

However, there's no point in saying "Cannot save with this file format" still,
as the error dialog has a header that says 'Unable to save project "%s"' and
the error strings raised by GES should be sufficient to use as secondary text.