Use boost::throw_exception. Refs #2094.

Still some throws left when rethrowing caught exceptions.

Thanks to Richard Smith.

[SVN r57604]

libs/iostreams

@@ -1 +1 @@
-Subproject commit ce5d78d3d21e42af6729364d7be401483cfa08dd
+Subproject commit d9af538532ec50b8e797e7f32a6602151b7d1bec