raft: server: pass *_aborted to set_exception call

This looks like a minor oversight, in `server_impl::abort` there are
multiple calls to `set_exception` on the different promises, only one of
them would not receive `*_aborted`.

raft/server.cc

@@ -1493,7 +1493,7 @@ future<> server_impl::abort(sstring reason) {
     }
 
     if (_state_change_promise) {
-        _state_change_promise->set_exception(stopped_error());
+        _state_change_promise->set_exception(stopped_error(*_aborted));
     }
 
     abort_snapshot_transfers();