Add action hook for failure to remove user favorite

[tw2113]

We have an action hook that fires when bp_update_user_meta( $user_id, 'bp_favorite_activities', $my_favs ) runs inside bp_activity_remove_user_favorite() and succeeds, but we don't have a corresponding one for if it fails.

If anyone wants to patch this, please add inline phpdocs for the hook, or ping me to remind me to come back to it :)

[DJPaul]

I'm not sure why a failure condition needs an action to fire. It's not something I think we usually do. Do you think we need one?

[DJPaul]

Closing this as invalid due to lack of counter-arguments from anyone else as to why we want to add this.