Print eventual lightning CSS parsing errors when the CSS matcher fail

[philipp-spiess]

When working on adding a test case with the slightly more complex arbitrary variants names:

[&[data-foo=1]+.bar]:underline

we noticed that the CSS formatting helpers were giving us unhelpful diff outputs that would make it hard to find the issue.

It turns out that we try to normalize the CSS with lightningcss and automatically fall back to prettier if lightning fails. This caused confusion and quite a bit of time to understand what's going on. In our case, the CSS being generated by Tailwind was not adding quotes around attribute selectors which caused lightning to fail but prettier to automatically insert these quotes.

To fix this, we now capture any eventual lightning error and include it when the test matcher is going to fail (so we keep the tests that currently rely on this behavior clean while adding useful debug information for new tests).

Here's an example of the new error message:

Error: Error formatting using Lightning CSS:

```css
    text-decoration-line: underline
}
.data-\[id\=0\]\:underline[data-id=0] {
                                    ^-- SyntaxError: Invalid value in attribute selector: Number { has_sign: false, value: 0.0, int_value: Some(0) }
    text-decoration-line: underline
}
```