vty_test_runner.py: raise exception when MSC socket connection fails

Instead of below error, raise an exception to describe what's happening.

Seen in a jenkins run on https://gerrit.osmocom.org/#/c/945/2:

ERROR: testBSCreload (__main__.TestVTYNAT)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "./vty_test_runner.py", line 787, in testBSCreload
    msc = nat_msc_test(self, ip, port)
  File "./vty_test_runner.py", line 1251, in nat_msc_test
    return conn
UnboundLocalError: local variable 'conn' referenced before assignment

Change-Id: Iae26e7345267a21aed0b108b089453832889c9fa
diff --git a/openbsc/tests/vty_test_runner.py b/openbsc/tests/vty_test_runner.py
index 9fe4c3e..f624fc9 100644
--- a/openbsc/tests/vty_test_runner.py
+++ b/openbsc/tests/vty_test_runner.py
@@ -1233,10 +1233,14 @@
     msc.listen(5)
     if (verbose):
         print "MSC is ready at " + ip
+    conn = None
     while "MSC is connected: 0" == x.vty.command("show msc connection"):
         conn, addr = msc.accept()
         if (verbose):
             print "MSC got connection from ", addr
+    if not conn:
+	raise Exception("VTY reports MSC is connected, but I haven't"
+			" connected yet: %r %r" % (ip, port))
     return conn
 
 def ipa_handle_small(x, verbose = False):