Provide more details on error when Markdown code block is misformatted

[parlough]

@MaryaBelanger I believe #5732 is caused by a code block without a language specified in your local branch in the /src/content/language/macros.md file. This PR makes that more obvious by providing an error explaining that.

I'll follow-up with a better error message, but wanted to make sure this actually was the issue you're facing. I'm also open to changing the requirement of a language if that's helpful or important to you. Thanks!

[dart-github-bot]

Visit the preview URL for this PR (updated for commit a46dc05):

https://dart-dev--pr5743-fix-5732-wixk2mlr.web.app

[MaryaBelanger]

Yes that was it! I think the error message reads perfectly, thanks for figuring it out.

How did you figure that out by the way? Reading the error log didn't seem to point to anything concrete

[parlough]

@MaryaBelanger I determined it roughly by:

I looked at the lines at the beginning of the stack trace:

[11ty] Original error stack trace: TypeError: Cannot read properties of null (reading 'length')
[11ty]     at markdown.renderer.rules.fence (file:///Users/mbelanger/Projects/dart.dev/dart-3/src/_11ty/plugins/highlight.js:49:37)

This line tried to use which was the result of token.info.match(/(\S+)\s?(.*?)$/m); (stored in splitTokenInfo) when parsing a Markdown code block(fence). So splitTokenInfo must have been null, meaning that regex didn't find any matches when parsing the opening fence line.