[plt-scheme] Catch an unknown method error

>  I think Noel's code is doing some hygiene by hand for you (why?).

Filipe had noted that his macro turned into a call to send/apply,
rather than send, and said this was a bit messy (you get a confusing
error message about send/apply rather than send if it goes wrong, for
instance).  I converted it to use send, which requires binding the
arguments to names to avoid multiple evaluation.  It serves as a
simple example of the technique for introducing new names (in this
case hygienically) into a syntax-case macro.

N.