Dump XML node if loading a source fails

This helps to debug issues, in case this comes up in a session
with many sources.

libs/ardour/session_state.cc

@@ -2585,6 +2585,7 @@ Session::XMLSourceFactory (const XMLNode& node)
 
 	catch (failed_constructor& err) {
 		error << string_compose (_("Found a sound file that cannot be used by %1. Talk to the programmers."), PROGRAM_NAME) << endmsg;
+		node.dump (std::cout, " Invalid Source: ");
 		return boost::shared_ptr<Source>();
 	}
 }