[exim] ${lsearch, defer upon a failure

Hello,

When I use ${lsearch, delivery is defered if the file exists but
couldnt' be read by exim, i.e.

2006-05-19 21:05:22 H=(foo.foo) [192.168.1.1] U=weirdo F=<> temporarily
rejected RCPT sthalik@???: failed to expand ACL string "${if and {
{ and { {!eq{$acl_m1}{}} {!match{$acl_m1}{/}} }}
{exists{/etc/mailconf/$acl_m1}}
{eq{on}{${lookup{helo}lsearch{/etc/mailconf/$acl_m1}}}}}}": failed to
open /etc/mailconf/sthalik for linear search: Permission denied
(euid=106 egid=102) inside "and{...}" condition

Is there a way to return false (instead of defer) after failures? If
not, are there any plans for inclusion of that feature in the following
versions?

-- sh