errors needs to be tostring()'d

[Daurnimator <quae@...>]

I just noticed that the __tostring metamethod is not invoked when an
error is at the top level in both luajit and lua:

error(setmetatable({},{__tostring=function(o) return "My error msg" end;})

This seems like bad beahviour to me.... makes custom error objects a bit harder.