`zine serve`: Error recovery

[plule]

Hello, when running zine serve and an error occurs, it would be nice if Zine would continue serving the last successful build, and display the new one once the issue is fixed.

Current behavior:

• ~~On missing md file: Error: Failed to read markdown file of \?\D:\dev\rust\test-zine\content\issue-1\1-first.md``, then exits~~
• ~~On invalid toml: error: invalid xx then exits~~
• On invalid date formatting, like pub_date = "2022--11-28": tokio-runtime-worker' panicked at 'called `Result::unwrap()` on an `Err` value: Failed to find the root zine.toml file',<snip>\src\serve.rs:39:14, then does not recover: This one is likely the most annoying, as an automatic process restart would not happen