Getting more useful errors

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 :-).
Get More Useful Errors
An alternative is to install the Swatter Add-on available at http://auctioneeraddon.com/dl/Swatter/