Fix and test the validation of template file

The validation message is created without proper import. Add it with new test.

Change-Id: Ia8a815e5f4b4c267e8a96faeebe8b5f9afe2b364
Partial-Bug: #1502937

toscaparser/tests/test_toscatpl.py

@@ -448,3 +448,11 @@ class ToscaTemplateTest(TestCase):
                                  'TestRsyslogType']
         self.assertItemsEqual(tosca.topology_template.custom_defs.keys(),
                               expected_custom_types)
+
+    def test_invalid_template_file(self):
+        template_file = 'invalid template file'
+        expected_msg = ('%s is not a valid file.' % template_file)
+        err = self.assertRaises(
+            ValueError,
+            lambda: ToscaTemplate(template_file, None, False))
+        self.assertEqual(expected_msg, err.__str__())

toscaparser/tosca_template.py

@@ -21,6 +21,7 @@ import toscaparser.imports
 from toscaparser.prereq.csar import CSAR
 from toscaparser.topology_template import TopologyTemplate
 from toscaparser.tpl_relationship_graph import ToscaGraph
+from toscaparser.utils.gettextutils import _
 import toscaparser.utils.yamlparser