config: use error() instead of fprintf(stderr, ...)

config: use error() instead of fprintf(stderr, ...)

The die() / error() / warning() helpers put a fatal: / error: /
warning: prefix in front of the error message they print describing
the message's severity, which users are likely to be accustomed to
seeing these days.

This change will also be useful when marking the message for
translation: the argument to error() includes no newline at the end,
so it is less fussy for translators to translate without lines running
together in the translated output.

While we're here, start the error messages with a lowercase letter to
match the usual typography of error messages.

A quick web search and a code search at codesearch.debian.net finds no
scripts trying to parse these error messages, so this change should be
safe.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>