error handling

[Victor Shoup]

So I've just released NTL v8, which is a library that builds on
the mpn-layer of GMP.  See: http://www.shoup.net/ntl/index.html

The main thing is that now NTL is "exception safe", modulo "exception safety"
in GMP (one can build a slower, fully exception-safe version of NTL that does
not rely on GMP).

So this increases a little bit the incentive to make GMP (or, at least, the
mpn-layer of GMP) exception safe.

Happy holdiays!

-Victor