[m-rev.] for review: Allow io.system_error values to be 0 or null.

[Julien Fischer]

On Mon, 29 Aug 2022, Peter Wang wrote:

> A Mercury foreign_proc needs to be able to return *something* for a
> system_error output argument when there is no error.
> On the C backends, it can return 0.
> On the C# and Java backends, it can return null.
>
> library/io.m:
>    Document 0 and null as valid values for system_error.
>
>    Handle null system_errors in system_error_exception_name and
>    get_exception_object_message.

That's fine.

Julien.