imap-send: don't use git_die_config() inside callback

imap-send: don't use git_die_config() inside callback

The point of git_die_config() is to let configset users mention the
file/line info for invalid config, like:

  if (!git_config_get_int("foo.bar", &value)) {
	if (!is_ok(value))
		git_die_config("foo.bar");
  }

Using it from within a config callback is unnecessary, because we can
simply return an error, at which point the config machinery will mention
the file/line of the offending variable. Worse, using git_die_config()
can actually produce the wrong location when the key is found in
multiple spots. For instance, with config like:

  [imap]
  host
  host = foo

we'll report the line number of the "host = foo" line, but the problem
is on the implicit-bool "host" line.

We can fix it by just returning an error code.

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