[Cfp-interest] FP_EXCEPTIONAL and subexceptions

[David Hough CFP]

There's a real problem with defining FP_EXCEPTIONAL without a list
of exceptions or subexceptions of interest.     In that case it has
to return a complete list of exceptions and subexceptions, whether you're
interested in them or not.     In general, executing code and determining
which subexceptions arose is going to be expensive and so you wouldn't
want to do that unless you cared.

Remember one of the goals here is that if the exception of interest does
NOT arise, then the typical use case is that the just-computed test
expression is actually the result you want.    So it needs to be ready
for use and efficient as well since it gets executed every time, unlike
the special case code for exceptions of interest.