Getting more useful errors

From WoWWiki

Add the following function to your Addon's LUA file:

function ErrorWithStack(msg)
   msg = msg.."\n"..debugstack()
   _ERRORMESSAGE(msg)
end

Add the following line to your Addon's Init function:

seterrorhandler(ErrorWithStack)

You're done :-).

An alternative is to install the Swatter Add-on available at http://auctioneeraddon.com/dl/Swatter/