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/tests/mod.rs

@@ -1,3 +1,4 @@
+use crate::value::Value;
 use builtin_macros::builtins;
 use pretty_assertions::assert_eq;
 use test_generator::test_resources;
@@ -57,19 +58,23 @@ fn eval_test(code_path: &str, expect_success: bool) {
     eval.builtins.extend(mock_builtins::builtins());
 
     let result = eval.evaluate();
-
-    if expect_success && !result.errors.is_empty() {
+    let failed = match result.value {
+        Some(Value::Catchable(_)) => true,
+        _ => !result.errors.is_empty(),
+    };
+    if expect_success && failed {
         panic!(
             "{code_path}: evaluation of eval-okay test should succeed, but failed with {:?}",
             result.errors,
         );
     }
 
-    if !expect_success && !result.errors.is_empty() {
+    if !expect_success && failed {
         return;
     }
 
-    let result_str = result.value.unwrap().to_string();
+    let value = result.value.unwrap();
+    let result_str = value.to_string();
 
     if let Ok(exp) = std::fs::read_to_string(exp_path) {
         if expect_success {