[Grace-core] New errors

[James Noble]

> I read the spec (and updated the name from catch()case()):

ta. updated svn to 

>  try()case() does not have an else clause, at least, not according to the spec.
> 
> I propose that we continue not to have one,

we don't need one because catch { _ -> ... } does fine.

>  and take it out of the parser.    If you don’t like that, we can discuss it further

that's a separate and different question.
how much does it hurt?

J