c-api: Avoid losing error context with instance traps (#5223)

This commit was a mistake from #5149

crates/c-api/src/instance.rs

@@ -98,13 +98,14 @@ pub(crate) fn handle_instantiate(
             *instance_ptr = i;
             None
         }
-        Err(e) => match e.downcast::<Trap>() {
-            Ok(trap) => {
-                *trap_ptr = Box::into_raw(Box::new(wasm_trap_t::new(trap.into())));
+        Err(e) => {
+            if e.is::<Trap>() {
+                *trap_ptr = Box::into_raw(Box::new(wasm_trap_t::new(e)));
                 None
+            } else {
+                Some(Box::new(e.into()))
             }
-            Err(e) => Some(Box::new(e.into())),
-        },
+        }
     }
 }