lint: enable errcheck (#5336)

## Description

Enable errcheck linter throughout the codebase

Closes: #5059

light/example_test.go

@@ -66,7 +66,9 @@ func ExampleClient_Update() {
 		stdlog.Fatal(err)
 	}
 	defer func() {
-		c.Cleanup()
+		if err := c.Cleanup(); err != nil {
+			stdlog.Fatal(err)
+		}
 	}()
 
 	time.Sleep(2 * time.Second)
@@ -135,7 +137,9 @@ func ExampleClient_VerifyLightBlockAtHeight() {
 		stdlog.Fatal(err)
 	}
 	defer func() {
-		c.Cleanup()
+		if err := c.Cleanup(); err != nil {
+			stdlog.Fatal(err)
+		}
 	}()
 
 	_, err = c.VerifyLightBlockAtHeight(3, time.Now())

light/rpc/client.go

@@ -55,7 +55,9 @@ func (c *Client) OnStart() error {
 
 func (c *Client) OnStop() {
 	if c.next.IsRunning() {
-		c.next.Stop()
+		if err := c.next.Stop(); err != nil {
+			c.Logger.Error("Error stopping on next", "err", err)
+		}
 	}
 }