boost::locale::generator::generate throws unhelpful error if iconv doesn't support UTF-16

[Rodger Combs <rodger.combs@…>]

This actually happens on some older libc's. You end up with an exception complaining about iconv not supporting ANSI_X3.4-1968, when in fact the issue is a lack of support for UTF-16, which should be either ignorable or reported more accurately. Gah, I'm too tired to explain the issue in more detail right now; poke me and I'll expand later if necessary.