Abort when no username is found

[timeless]

On Tue, Oct 20, 2009 at 1:38 AM, Martin Geisler <mg at lazybytes.net> wrote:
>  Please specify a username.

>  please specify a username, see "hg help hgrc"

Please specify a username (ui.username); see "hg help hgrc"

I'd expect hg help hgrc to explain how to add things but not have an
exhaustive list of sections/values, so not indicating the value in the
message is problematic.