Add a comment about deprecated function

Author: atsmtat

src/helpers.rs

@@ -664,6 +664,9 @@ where
 }
 
 pub fn isolation_error(name: &str) -> InterpResult<'static> {
+    // FIXME: This function has been deprecated. It's only used for
+    // the ops which are not converted to return a proper error code
+    // in isolation.
     throw_machine_stop!(TerminationInfo::UnsupportedInIsolation(format!(
         "{} not available when isolation is enabled",
         name,