Enable some linters (#3129)

Mostly those that did not require much work.

From #2960

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

agent/state.go

@@ -74,8 +74,11 @@ func (s *State) Healthy() bool {
 
 func (s *State) WriteTo(w io.Writer) (int64, error) {
 	s.Lock()
-	out, _ := json.Marshal(s)
+	out, err := json.Marshal(s)
 	s.Unlock()
+	if err != nil {
+		return 0, err
+	}
 	ret, err := w.Write(out)
 	return int64(ret), err
 }