这是一个安装了 OmniMarkupPreviewer 插件后，又缺少了依赖（比如重装系统）后才会出现的问题。见这篇文章。
错误现象：
Error: 404 Not Found
Quick Fix 1: Remove Strikethrough Extension
Sublime Text > Preferences > Package Settings > OmniMarkupPreviewer > Settings - User
paste the following to remove the strikeout package.
{
"renderer_options-MarkdownRenderer": {
"extensions": ["tables", "fenced_code", "codehilite"]
}
}