oap_client: reject all messages in disabled/uninitialized state

Fixes the bug indicated in oap_client_test.c: adjust to actually expect the
proper behavior.

Also adjust for modified return value for message rejection. Instead of -1,
just expect < 0.

Adjust experr for new error messages.

Related: OS#1592
Change-Id: I16165d228653e8a2689f9df94b77b470c06480c6
diff --git a/openbsc/src/libcommon/gsup_client.c b/openbsc/src/libcommon/gsup_client.c
index 0360e0a..4c1fe61 100644
--- a/openbsc/src/libcommon/gsup_client.c
+++ b/openbsc/src/libcommon/gsup_client.c
@@ -155,6 +155,7 @@
 	int rc;
 	struct msgb *msg_tx;
 
+	/* If the oap_state is disabled, this will reject the messages. */
 	rc = oap_client_handle(&gsupc->oap_state, msg_rx, &msg_tx);
 	msgb_free(msg_rx);
 	if (rc < 0)