fix_return_values() trick taken from http://lua-users.org/wiki/FinalizedExceptions. This page also explains the idea behind the thing.
This pure Lua version can be nice if you want to avoid cross compilation, or you want to understand how newtry, protect and try works if you are not so fluent in the Lua C API. (Both applied to me).
Note: Unlike lua-try, I chose to use an unique metatable instead of an unique table key to tag newtry errors. This saves one if-clause in protect(). But then try/co.lua had to be modified.
The short story is that you can use can newtry(finalizer) to make a function wrapper that can be applied on functions that uses the idiomatic Lua "return nil, error_msg" convention on exceptions. If exception (that is that the first return value is falsy), then finalizer() is run (cleaning up your resources), and Lua error(error_msg) is raised. The error_msg is wrapped and tagged so that it can be caught by try.protect()(...).
Then protect(func) can be used to wrap function func so that it catches errors made from underlying try.newtry()(...) exceptions. If the error() came from a try.newtry()(...) exception, then the exception is converted back to idiomatic "return nil, error_msg". Other errors are passed on by calling Lua error() again.