ident: reword empty ident error message

ident: reword empty ident error message

There's on point in printing the name, since it is by
definition the empty string if we have reached this code
path. Instead, let's be more clear that we are complaining
about the empty name, but still show the email address that
it is attached to (since that may provide some context to
the user).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>