fixed spelling

move FrechetDeriv from the Library to HOL/Deriv; base DERIV on FDERIV and both derivatives allow a restricted support set; FDERIV is now an abbreviation of has_derivative

remove the within-filter, replace "at" by "at _ within UNIV" (This allows to remove a couple of redundant lemmas)

improved printing of exception CTERM (see also d0f0f37ec346);

prefer pretty_exn where possible -- NB: low-level General.exnMessage may still be used elsewhere (e.g. by the ML compiler itself);

more defensive representation of forced break within PolyML.PrettyBreak -- avoid accidental blowup if low-level operations are used, notably PolyML.makestring or its variant General.exnMessage;