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.cpp

@@ -322,7 +322,7 @@ void MeshService::sendOurPosition(NodeNum dest, bool wantReplies)
     sendToMesh(p);
 }
 
-void MeshService::onGPSChanged(void *unused)
+int MeshService::onGPSChanged(void *unused)
 {
     DEBUG_MSG("got gps notify\n");
 
@@ -354,4 +354,6 @@ void MeshService::onGPSChanged(void *unused)
 
         releaseToPool(p);
     }
+
+    return 0;
 }