libmsc/gsm_09_11.c: send GSUP PROS_SS ERROR message when needed

OsmoMSC should notify the remote SS/USSD entity if:

  - received GSUP message has unexpected session state;
  - received GSUP message has unknown session ID;
  - received GSUP message missing mandatory IE(s);
  - NCSS transaction establishment failed;
  - NCSS message delivery failed.

Change-Id: Ief9f8a197b0860072b671edfc55180f619860d9d
Related: (TTCN-3) Ie267ee174c5061cd3fc102a2824abe03d73f3aac
Related: OS#2931
1 file changed