fix(tvix/eval): fix b/281 by adding Value::Catchable

This commit makes catchable errors a variant of Value.

The main downside of this approach is that we lose the ability to
use Rust's `?` syntax for propagating catchable errors.

Change-Id: Ibe89438d8a70dcec29e016df692b5bf88a5cad13
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9289
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: Adam Joseph <adam@westernsemico.com>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/to_xml.rs

@@ -134,6 +134,10 @@ fn value_variant_to_xml<W: Write>(w: &mut EventWriter<W>, value: &Value) -> Resu
                 metadata: Some(Rc::new(value.clone())),
             })
         }
+
+        Value::Catchable(_) => {
+            panic!("tvix bug: value_to_xml() called on a value which had not been deep-forced")
+        }
     }?;
 
     Ok(())