[m-rev.] for post-commit review: better diagnostics for foreign_*enum pragmas

[Julien Fischer]

On Mon, 5 Jun 2017, Zoltan Somogyi wrote:

> For post-commit review by Julien, since he did much of the
> work on foreign enums, and he is as familiar with reserve_tag
> pragmas as anyone else.

That looks fine.

Julien.