Bad Error Message if Missing sized in struct forall.

[ajbeach]

The code does not support the following code:

forall(dtype T | sized(T))
struct wrap {
    T get;
};

forall(dtype U)
struct wrap_ptr {
    wrap(U) * get;
};

And even if we never support this the error message is terrible. The compiler spits about over a half dozen errors, most of which refer to missing members in the wrap_ptr type. The fix is to add | sized(U) to the second forall list, but the error message does not hint at that at all.