Add the missing error_exit function to ha_backend_storage

otherwise hilarious things happen when tries are exhausted:

```
Tue, 09 Dec 2014 20:39:58 +0000: Attempting to become DRBD secondary - attempts left 1
/var/opt/opscode/drbd/data is a mountpoint
/var/opt/opscode/drbd/data: 29583c 29586c 29587c 29588c 29589c 29590c
umount2: Device or resource busy
umount: /var/opt/opscode/drbd/data: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
umount2: Device or resource busy
0: State change failed: (-12) Device is held open by someone
Command 'drbdsetup-84 secondary 0' terminated with exit code 11
/var/opt/opscode/keepalived/bin/ha_backend_storage: line 40: error_exit: command not found
Attempting to become DRBD secondary - attempts left 0
Tue, 09 Dec 2014 20:39:59 +0000: Attempting to become DRBD secondary - attempts left 0
/var/opt/opscode/drbd/data is a mountpoint
/var/opt/opscode/drbd/data: 29603c 29607c 29608c 29609c 29610c 29611c
umount2: Device or resource busy
umount: /var/opt/opscode/drbd/data: device is busy.
        (In some cases useful info about processes that use
         the device is found by lsof(8) or fuser(1))
umount2: Device or resource busy
0: State change failed: (-12) Device is held open by someone
Command 'drbdsetup-84 secondary 0' terminated with exit code 11
Attempting to become DRBD secondary - attempts left -1
Tue, 09 Dec 2014 20:40:00 +0000: Attempting to become DRBD secondary - attempts left -1
... ad nauseum...
```

because of this
```
/var/opt/opscode/keepalived/bin/ha_backend_storage: line 40: error_exit: command not found
```