Add facilities to report and enrich error messages.
The idea is that a given error should be reported only once to the user. At the moment, for example, we can get: - "no suitable access module" (printed by module_Need) - "unable to create access" (printed by input/access) - "unable to open stream" (printed by input/input) - ... The facilities provided here are: * msg_StackSet( code, message ) : "throw" a new error * msg_StackAdd( message ) : Append a message allowing to trace the message * msg_StackMsg() : Retrieve the message So, in the previous example, module_Need would msg_StackSet( code, "no suitable access module"), then input/input would add "unable to create access" and finally, input/input can print the "unable to open stream: unable to create access: no suitable module" message. The three functions are context-free, you don't need a vlc_object, so that we can add verbose error reporting everywhere.