Confusing error on unrecognized language

[porglezomp]

If I use an unrecognized syntax, for instance:

File index.md:

```asdfl
```

Then the error is confusing.

Language does not exist asdfl
Problem writing Eleventy templates: (more in DEBUG output)
> Having trouble rendering njk (and markdown) template ./index.md

`TemplateContentRenderError` was thrown
> Cannot read property 'rest' of undefined

The first line mentions that the language doesn't exist, but I didn't realize for quite a while that that warning was the cause of the later fatal error.

Expected Behavior:
Unrecognized languages should give a warning and then simply fail to highlight that syntax.

[zachleat]

That console.warn Language does not exist asdfl actually comes from Prism, unfortunately. PrismJS/prism#2145

But we can clean up the error message.