Internal and/or misleading error messages around throwing / virtual-dispatched iterators

[e-kayrakli]

use List;

class C {
  iter foo() throws {
    for i in 1..10 do yield i;
  }
}

class D: C {
  override iter foo() throws  {
    for i in 1..10 do yield 2*i;
  }
}

var l = new list(shared C?);

l.pushBack(new shared C());
l.pushBack(new shared D());

for item in l {
  for x in item!.foo() {
    writeln(x);
  }
}

Above, I have a simple class hierarchy with an overridden iterator. If I want to make these iterators throw, I get error: throwing non-inlined iterators are not yet supported. Where my guess is that these are not inlined because they are virtually dispatched. The error message isn't very helpful. We can simply blanket prohibit throwing overridden iterators.

---

As a related issue, if you remove throws from C.foo(), you get an internal compiler error. It looks like throws is part of a proc's interface when it is overridden (if parent's throws, children's must throw, too). And failure to do that is reflected with a nice error message. We should have the same for the iterator case.