Removed boost::exception::what, to avoid ambiguity in user types deriving from both std::exception and boost::exception.

[SVN r46930]

test/Jamfile.v2

@@ -29,7 +29,6 @@ run throw_exception_test.cpp helper2.cpp ;
 run errno_test.cpp ;
 run error_info_test.cpp ;
 run diagnostic_information_test.cpp ;
-run what_test.cpp ;
 compile-fail exception_fail.cpp ;
 compile-fail throw_exception_fail.cpp ;