Be more informative when JSONpaths do not match

When the YAML files uses JSONpaths that do not or cannot match we
need to know which patch it was that didn't match, otherwise we just
get a rather lame error message that is not useful information for
fixing the problem.

gabbi/case.py

@@ -81,7 +81,9 @@ class HTTPTestCase(testtools.TestCase):
             self.json_data = response_data
             for path in json_paths:
                 match = self._extract_json_path_value(response_data, path)
-                self.assertEqual(json_paths[path], match)
+                self.assertEqual(json_paths[path], match,
+                                 'Unable to match %s as %s'
+                                 % (path, json_paths[path]))
 
     def _decode_content(self, response, content):
         """Decode content to a proper string."""
@@ -107,7 +109,12 @@ class HTTPTestCase(testtools.TestCase):
         """
         path_expr = jsonpath_rw.parse(path)
         matches = [match.value for match in path_expr.find(data)]
-        return matches[0]
+        try:
+            return matches[0]
+        except IndexError:
+            raise ValueError(
+                    "JSONPath '%s' failed to match on data: '%s'"
+                    % (path, data))
 
     def _load_data_file(self, filename):
         """Read a file from the current test directory."""