In Lua, one can throw any type as an error. If you throw a string, Lua adds some stack info. In Luan, whatever you throw becomes a message of an Exception and the Exception is thrown. I don't like this because when you catch the exception in Luan, it is a Java object that you can't really access. I don't like Lua's approach either.
I think the right approach is for an error to be a table. The message would be in the table. The stack trace would just be a list of stack trace elements in the table. And the table would have a metatable containing __to_string to print out the whole error in a nice way. Of course I would still throw an Exception internally, but the Exception would have a reference to the table and the table would have a reference to the exception. Luan code would only see the table, but one could re-throw the table since it contains a reference to the exception.