[plt-scheme] v208 -> v299

I finally got around to upgrading to v299 -- I couldn't really avoid
it since the exp tag changed.  The first thing I ran into was that
`not-break-exn?' disappeared, and since it took me a while to track
down a solution (had to use Google site search), I figured I'd mention
it here: use `exn:fail?' instead.

The PLT Release Chronology page is several months out of date:
http://download.plt-scheme.org/chronology/
Is there somewhere else that the release notes have been posted
(besides the mailing list archive)?  It would be nice if there were a
single page somewhere that collected the text of all the v299 release
notes, preferably in reverse chronological order... but I'm not
volunteering to make one.

--dougo at place.org