lua: Print traceback on startup errors

We do some special magic so that we can have tracebacks on errors messages.
However, the code for parsing the rc.lua called it without this magic and thus
errors didn't have tracebacks.

This is bad, because if something goes wrong in e.g. wibox.widget.textbox, you
don't really have any clue where this error is coming from.

Fix this by adding our "print traceback on error"-magic here, too.

Signed-off-by: Uli Schlachter <psychon@znc.in>