Don't throw an exception for unknown code block languages

[aaronpk]

In my source content, I have:

```HTTP
stuff
```

This results in a Javascript error:

example.html:28 Uncaught Error: Unknown language: "HTTP"
Please report this to https://github.com/chjj/marked.

If an unknown language is encountered, it should be treated as plain text rather than throwing an exception.

[Feder1co5oave]

I'm pretty sure this is highlight.js throwing the exception, and marked just reports it. What options are you using?

[acorbel]

To prevent this exception, I did:

markedProvider.setOptions({
            gfm: true,
            tables: true,
            breaks: false,
            pedantic: false,
            sanitize: true, // if false -> allow plain old HTML ;)
            smartLists: true,
            smartypants: false,
            highlight: function (code, lang) {
              if (lang && hljs.getLanguage(lang)) {
                return hljs.highlight(lang, code, true).value;
              } else {
                return hljs.highlightAuto(code).value;
              }
            }
          });

Hope it helps.

[joshbruce]

#983