Changes

SMS-over-GSUP: send network-originated MT-forwardSM-Err

When SMS-over-GSUP is enabled, there's currently no indication of
failed MT SMS delivery due to paging timeout or connection issues.
In such cases, the SMSC that initiated the MT-forwardSM-Req receives
no response at all.  This leaves the SMSC with no clear criterion
for retrying delivery other than a generic timeout.

So far, MT-forwardSM-Err was only sent when the MS/UE explicitly
indicated an error (via RP-ERROR).  This patch extends its use to
also cover situations where no RP-ACK or RP-ERROR is received,
allowing the network to notify the SMSC of delivery failure
due to lower-layer issues.

The key difference between the MS/UE-originated and network-originated
MT-forwardSM-Err messages is presence/absence of the following IEs:

* SM-RP-Cause IE -- indicated by the MS/UE,
* Cause IE -- indicated by the network.

Either of these IEs shall always be present.

Change-Id: I51d92752471147e6d21a5059bebb0702b32642a5
Related: osmo-ttcn3-hacks.git Ief16753783e044911f1e3cef8d7012810d709e61
Related: osmo-ttcn3-hacks.git Ic27d943645fdff8d34f958e3bf16c8ee6e8368d2
Related: SYS#7323