code: def _check_backends(self):
          
          backends = self.backends_params.keys()
          for b in self.roles.get_backends():
              if b not in backends:
                  raise MissingBackend(b, 'role')
          for b in self.attributes.get_backends():
              if b not in backends:
                  raise MissingBackend(b, 'attribute')
docs: Check that every backend in roles and attributes
          is declared in main configuration