Detect double monitoring of servers

Adding a server to multiple monitors is forbidden. This should be detected
and reported to the end user.

The information provided by the config_runtime system to the client isn't
as detailed as it could be. Some sort of an error message stack should be
added so that client facing interfaces could properly report the reason
for the failure. Currently the only way to detect the reason of the
failure is to parse the log files.

include/maxscale/service.h

@@ -240,7 +240,7 @@ bool serviceStartListener(SERVICE *service, const char *name);
 SERVICE* service_find(const char *name);
 
 // TODO: Change binlogrouter to use the functions in config_runtime.h
-void serviceAddBackend(SERVICE *service, SERVER *server);
+bool serviceAddBackend(SERVICE *service, SERVER *server);
 
 /**
  * @brief Check if a service uses a server