[m-rev.] diff: Fix use of MR_strerror.

[Julien Fischer]

On Thu, 31 Oct 2019, Peter Wang wrote:

> Also, error_to_cerrno returning -1 for unknown(Err, Msg) seems like a
> weird choice.

Yes, looking at that again, it's not really a great choice.  I'll change
it to be semidet and only succeed for non-unknown errors.

Julien.