Fixed errcheck issues in server/channels/api4/license_local.go (#28558)

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

server/channels/api4/license_local.go

@@ -54,7 +54,10 @@ func localAddLicense(c *Context, w http.ResponseWriter, r *http.Request) {
 	defer file.Close()
 
 	buf := bytes.NewBuffer(nil)
-	io.Copy(buf, file)
+	if _, err := io.Copy(buf, file); err != nil {
+		c.Err = model.NewAppError("addLicense", "api.license.add_license.copy.app_error", nil, "", http.StatusInternalServerError).Wrap(err)
+		return
+	}
 
 	license, appErr := c.App.Srv().SaveLicense(buf.Bytes())
 	if appErr != nil {