ident: give "please tell me" message upon useConfigOnly error

ident: give "please tell me" message upon useConfigOnly error

The env_hint message applies perfectly to the case when
user.useConfigOnly is set and at least one of the user.name and the
user.email are not provided.

Additionally, use a less descriptive error message to discourage
users from disabling user.useConfigOnly configuration variable to
work around this error condition.  We want to encourage them to set
user.name or user.email instead.

Signed-off-by: Marios Titas <redneb@gmx.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>