[m-rev.] for post-commit review: improve error messages for bad pragmas

[Julien Fischer]

On Sun, 23 Aug 2020, Zoltan Somogyi wrote:

> I found the need for improvement when I tried to compile
> the pragma memos updated with disable_warning_if_ignored
> with a compiler that did not recognize it.
>
> For review by anyone.

That looks fine.

Julien.