Replace `unreachable` with `bail` in `finalize_css` (#8694)
### Description
@rauchg reported:
```
thread 'tokio-runtime-worker' panicked at /Users/geist/.cargo/git/checkouts/turbo-df7a549334890fa5/2af6b12/crates/turbopack-css/src/process.rs:439:22:
internal error: entered unreachable code: this case should be filtered out while parsing
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```
(https://vercel.slack.com/archives/C01A2M9R8RZ/p1720449958135569)
A review of the code confirms that this should be unreachable under normal circumstances:
> There's a check here that `FileContent::Content` exists before constructing the `ParseCssResult::Ok` that's needed to reach the line that contains the `unreachable!` assertion: https://github.com/vercel/turbo/blob/main/crates/turbopack-css/src/process.rs#L491
But there's no guarantee that the view of the world is strongly consistent across multiple `Vc`s inside most tasks. Some `Vc`s may no longer exist due to invalidation (or cache eviction once that's enabled) while this task was running.
There is a guarantee that at the top-level of the application that there's strong (eventual) consistency, so we can be confident that this `bail!` won't bubble all the way up.
### Testing Instructions
`cargo nextest r -p 'turbopack-*'`
