config: turn die_on_error into caller-facing enum

config: turn die_on_error into caller-facing enum

The config code has a die_on_error flag, which lets us emit
an error() instead of dying when we see a bogus config file.
But there's no way for a caller of the config code to set
this: it's auto-set based on whether we're reading a file or
a blob.

Instead, let's add it to the config_options struct. When
it's not set (or we have no options) we'll continue to fall
back to the existing file/blob behavior.

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