Add a "mandatory" flag to WebLabServer
When the application server or the bus does not start, the weblab starting process stops early. However, when you add some custom servers (e.g. fuseki, heritrix), and that an error occurred that prevent one of them from starting. It does not impact the starting process. It would be better to have an attribute "mandatory" (being false by default), enabling to force the launcher to stop if an error occurred when a given server.