blog: doc usage of anyhow!

Since https://github.com/extendr/extendr/pull/973 returning `anyhow::Result<>` works without any modification.

I think this is a huge quality of life improvement. We can make a patch release with these features and document it?

For example:

```rust
use anyhow::bail;
use extendr_api::prelude::*;

#[extendr]
fn can_we_anyhow() -> anyhow::Result<()> {
bail!("This is an error");
Ok(())
}
```

lets us return a nice error