library: Ensure setverdict(fail) is followed by mtc.stop

This will prevent subsequent failures from overwriting the verdict so we
can easily see the root cause of the test failure.

Using testcase.stop instead for errors internal to our test
infrastructure to mark them as test errors instead of failed.

Change-Id: Idc6819aaf0b01e70c38fad828dd44dcec6bdd778
diff --git a/library/GSM_Types.ttcn b/library/GSM_Types.ttcn
index 2000b4a..59ccb76 100644
--- a/library/GSM_Types.ttcn
+++ b/library/GSM_Types.ttcn
@@ -34,6 +34,7 @@
 		case (CS3) { return 52 }
 		case else {
 			setverdict(fail, "Invalid GPRS CS ", cs);
+			mtc.stop;
 			return -1;
 			}
 		}