Allow engine parser errors to be handled

[ViciousBadger]

In engines/toml.rs (and similar in the other engines)

impl Engine for TOML {
    fn parse(content: &str) -> Pod {
        match toml::from_str::<Value>(content) {
            Ok(value) => value.into(),
            Err(_) => Pod::Null,
        }
    }
}

This makes it more difficult to troubleshoot issues with front matter parsing, since any internal error thrown by the parser is completely hidden. It would be nice if the Pod enum instead contained an error state containing whatever error type the internal parser uses. (Or something like that, I'm no expert)

[yuchanns]

SGTM.