[MM-61517] Fix errcheck issues in channels/app/plugin_api_tests/manual.test_http_hijack_plugin (#29190)

Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/.golangci.yml

@@ -122,7 +122,6 @@ issues:
         channels/app/platform/web_conn.go|\
         channels/app/platform/web_hub.go|\
         channels/app/platform/web_hub_test.go|\
-        channels/app/plugin_api_tests/manual.test_http_hijack_plugin/main.go|\
         channels/app/plugin_api_tests/manual.test_serve_metrics_plugin/main.go|\
         channels/app/plugin_api_tests/test_update_user_auth_plugin/main.go|\
         channels/app/plugin_install.go|\

server/channels/app/plugin_api_tests/manual.test_http_hijack_plugin/main.go

@@ -7,6 +7,7 @@ import (
 	"net/http"
 
 	"github.com/mattermost/mattermost/server/public/plugin"
+	"github.com/mattermost/mattermost/server/public/shared/mlog"
 )
 
 type Plugin struct {
@@ -26,8 +27,18 @@ func (p *Plugin) ServeHTTP(_ *plugin.Context, w http.ResponseWriter, _ *http.Req
 		return
 	}
 
-	conn.Write([]byte("HTTP/1.1 200\n\nOK"))
-	conn.Close()
+	_, err = conn.Write([]byte("HTTP/1.1 200\n\nOK"))
+	if err != nil {
+		mlog.Error("Failed to write to connection", mlog.Err(err))
+		w.WriteHeader(http.StatusInternalServerError)
+		return
+	}
+	err = conn.Close()
+	if err != nil {
+		mlog.Error("Failed to close connection", mlog.Err(err))
+		w.WriteHeader(http.StatusInternalServerError)
+		return
+	}
 }
 
 func main() {