Fix a misuse of errno.

Fix a misuse of errno.

In menu_save_event, we checked ctx.error to see if an errno value had
been left in it by the savefile_write callback, but if so, then we
were passing the _current_ value of errno to strerror() in place of
the saved value in ctx.error.

This may well have been benign, but I spotted it in an eyeball review
just now and thought I'd better fix it before it bit anyone.