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/esp32/MeshBluetoothService.cpp

@@ -222,10 +222,11 @@ class FromNumCharacteristic : public CallbackCharacteristic, public Observer<uin
     }
 
     /// If the mesh service tells us fromNum has changed, tell the phone
-    virtual void onNotify(uint32_t newValue)
+    virtual int onNotify(uint32_t newValue)
     {
         setValue(newValue);
         notify();
+        return 0;
     }
 };