failed conversion of enums results in error dialog, in debug mode

The make_enum macro header now includes wml_exception.hpp, and
game_config. We use the FAIL macro to launch a transient
twml_exception dialog when an enum is casted from an illegal
string value. This means that when enums in our C++ classes are
initialized using lexical_cast<enum_type> (cfg["attribute"]),
we will automatically give error reports for bad values.
update changelog
Merge branch 'master' of git://github.com/wesnoth/wesnoth