This function is just a thin wrapper for the "_gently" form
of the function. But the gently form is designed to feed
builtin/config.c, which passes our return code directly to
its exit status, and thus uses positive error values for
some cases. We check only negative values, meaning we would
fail to die in some cases (e.g., a malformed key).

This may or may not be triggerable in practice; we tend to
use this non-gentle form only when setting internal
variables, which would not have malformed keys.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>