rsl: If CHAN ACT or MODE MODIF fails, send respective NACK
rsl: If CHAN ACT or MODE MODIF fails, send respective NACK

The existign code only sent an ERROR REPORT, but it failed to actually
send a proper NACK to the related request.  This is confusing, as the
operation should always be ACKed or NACKed, and not simply result in no
response.

Change-Id: Ic374a8e5e239ffe37082a54cdb94cb6ac9723e83
Closes: OS#3254

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts