L2SS-874 added missing MIB test

tangostationcontrol/tangostationcontrol/test/clients/test_snmp_client.py

 from pysnmp import hlapi
-from pysnmp.smi import view
+from pysnmp.smi import view, error
 from pysnmp.smi.rfc1902 import ObjectIdentity
 from os import path
 import numpy
@@ -260,3 +260,11 @@ class TestMibLoading(base.TestCase):
 
         # get the numerical value
         self.assertEqual(int(obj_T[1]), testNamedValue_value)
+
+    def test_mib_missing(self):
+        abs_dir = path.dirname(__file__) + "/" + self.REL_DIR + "/"
+        loader = mib_loader(abs_dir)
+
+        with self.assertRaises(error.MibNotFoundError):
+            loader.load_pymib("FAKE-MIB")
+