chore: explain a todo in the machine

crates/uplc/src/machine.rs

@@ -242,7 +242,7 @@ impl Machine {
                     )),
                     None => todo!(),
                 },
-                _ => todo!(),
+                _ => todo!("return a proper evaluation error"),
             },
         }
     }

crates/uplc/tests/conformance.rs

@@ -28,7 +28,9 @@ fn actual_evaluation_result(file: &Path) -> Result<Program<Name>, String> {
 
     let program = parser::program(&code).map_err(|_| PARSE_ERROR.to_string())?;
 
-    let program: Program<NamedDeBruijn> = program.try_into().unwrap();
+    let program: Program<NamedDeBruijn> = program
+        .try_into()
+        .map_err(|_| EVALUATION_FAILURE.to_string())?;
 
     let version = program.version;