[Freeciv-Dev] (PR#13038) need better feedback when script function doesn't exist

["Jason Short" <jdorje@xxxxxxxxxxxxxxxxxxxxx>]

<URL: http://bugs.freeciv.org/Ticket/Display.html?id=13038 >

1: Error in script function "hut_enter_tutorial_callback":
1: attempt to call a nil value

the error here is that the hut_enter_tutorial_callback function doesn't
exist (it's hut_entered_tutorial_callback).  But there's nowhere to know
this from the message.  I spent 15 minutes looking for a nil reference
or type mismatch inside the function.

-jason