message: Builder: Include provided failure reason in error messages.
diff: @@ -1884,7 +1884,9 @@ class Builder:
  self.__builder = builder
  def __str__(self):
  - return '%s failed' % self.builder
  + ctx = self.__context__
  + return '{} failed{}'.format(
  + self.builder, (': ' + str(ctx)) if ctx is not None else '')
  @property
  def builder(self):
  