Raise the correct exception

mayan/apps/ocr/backends/__init__.py

@@ -1,3 +1,3 @@
 class BackendBase(object):
-    def execute(input_filename, language=None):
+    def execute(input_filename, language=None):  # NOQA
-        raise NotImplemented
+        raise NotImplementedError

mayan/apps/statistics/classes.py

@@ -51,7 +51,7 @@ class Statistic(object):
         return unicode(self.label)
 
     def get_results(self, *args, **kwargs):
-        return NotImplemented
+        return NotImplementedError
 
     @property
     def id(self):