LPEG error location display

[Thomas Harning <harningt@...>]

I'm working on enhancing my LuaJSON library that uses LPEG for  
decoding such that it can output the location of invalid JSON data.

I tried throwing in the (-1 * lpeg.P(er)) at the end of the expression  
and that only seems to work when there's extraneous characters at the  
end of the valid data...  If I insert invalid characters inside the  
data, it fails without calling the error function.

This seems to make sense, but I can't quite wrap my head around how to  
detect&display errors in a sane way.