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/value/attrs.rs

@@ -393,7 +393,7 @@ impl NixAttrs {
             // /another/ set with a __toString attr.
             let s = generators::request_string_coerce(co, result, kind).await;
 
-            return Some(s);
+            return Some(s.ok()?);
         }
 
         None