Merging from trunk: new function diagnostic_information_what, and mutable error info access.

[SVN r56477]

test/error_info_const_fail.cpp

@@ -0,0 +1,14 @@
+//Copyright (c) 2006-2009 Emil Dotchevski and Reverge Studios, Inc.
+
+//Distributed under the Boost Software License, Version 1.0. (See accompanying
+//file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/exception/get_error_info.hpp>
+
+typedef boost::error_info<struct foo_,int> foo;
+
+void
+test( boost::exception const * e )
+    {
+    ++*boost::get_error_info<foo>(*e);
+    }