allow observers to return an error code to abort further processing

Will allow me to use observers to generalize the various hooks
that need to run to preflight sleep entry.

src/MeshService.h

@@ -80,7 +80,8 @@ class MeshService
     void sendToMesh(MeshPacket *p);
 
     /// Called when our gps position has changed - updates nodedb and sends Location message out into the mesh
-    void onGPSChanged(void *arg);
+    /// returns 0 to allow futher processing
+    int onGPSChanged(void *arg);
 
     /// handle all the packets that just arrived from the mesh radio
     void handleFromRadio();