Better error messages...

err

Problem:
--------

You tried to use the random number generator effect when it wasn't available.

New error message:
------------------

Expr.idr:31:6:When elaborating right hand side of eval:
Can't solve goal 
	SubList [RND] [(EXCEPTION String),STDIO,(STATE (List (String, Integer)))]
	
Old error message:
------------------

Expr.idr:31:6:When elaborating right hand side of eval:
Can't unify
	Eff IO Integer [RND] (\ result => [RND])
with
	Eff IO a [(MkEff () (Exception String)),(MkEff () StdIO),(MkEff (List (String, Integer)) State)] xs'

Specifically:
	Can't unify
		Integer
	with
		()

I think that's progress :)