[m-rev.] for review: improved error messages for unexpected ::mode suffixes

[Zoltan Somogyi]

I think that is fine, except for one minor thing.
The .err_exp file now contains wording like this:

  In type definition: in the first argument:

The thing is, a type definition has more than one part
that has arguments: the type constructor may have arguments,
in eqv type definitions the equivalent type has arguments,
in du types, the function symbols have arguments, etc.
So I think the ContextPieces need have added to them
the identity of the thing whose arguments you are parsing
before you parse the arguments. There was an XXX about
this, but the missing update didn't matter as much,
because we never added anything to the too-short context;
now we do.

Zoltan.