Improve error handling

[henix]

There are 2 styles of error handling in Lua world:

1. Returns (nil, err): The caller must handle return value.
2. Calls error / assert (like exceptions in other languages): The caller must use pcall to handle error.

Now I prefer the 2nd style. Anyway, the document should describe how slt2 handle errors.

Compile-time error:

1. start_tag / end_tag missing
2. include file not found