[exim] test database conn to prevent rejecting messages

I am writing an ACL that a few of my relay servers will use. All is fine
with code. However if the central database server can not be connected to I
want to disable the ACL to prevent messages from being rejected.

Right now this is what happens if DB conn fails (example of stopping mysqld
service)

SMTP>> 451 Temporary local problem - please try later451 Temporary

local problem - please try later
LOG: MAIN REJECT
  H=localhost [127.0.0.1] F=<root@???> temporarily
rejected after DATA: MYSQL     connection failed: Can't connect to
local MySQL server through socket '/var/lib/mysql/mysql.sock'    (2)

Thanks