fix dowhy_ not existance issue

[louis-she]

1. object has no __getattr__ attribute so calling super().__getattr__ is wrong, should change it to getattr(self, name).
2. dowhy_ is not exist until fit is called, just have a more intuitive error message when someone calls methods without fitting.

[kbattocchi]

Thanks for the contribution! I have two requests:

1. This repo requires all commits to be signed off (which yours isn't), which is causing the DCO check to fail - please amend your commit to sign off on it (there are detailed instructions that you can see if you look at the details of the failing DCO check).
2. Please add a test case that fails with the current version of the code but passes with your changes.

[louis-she]

if without this patch,

LinearDRLearner().dowhy.refute_estimate(method_name="random_common_cause", num_simulations=3)
# or
LinearDRLearner().dowhy._estimator_object

will raise

Traceback (most recent call last):
  File "/Users/louisshe/Downloads/EconML/econml/tests/test_dowhy.py", line 100, in test_dowhy_without_fit
    LinearDRLearner().dowhy.refute_estimate(method_name="random_common_cause", num_simulations=3)
  File "/Users/louisshe/Downloads/EconML/econml/dowhy.py", line 220, in refute_estimate
    return self.dowhy_.refute_estimate(
  File "/Users/louisshe/Downloads/EconML/econml/dowhy.py", line 236, in __getattr__
    return super().__getattr__(attr)
AttributeError: 'super' object has no attribute '__getattr__'

super() is object because the defination class DoWhyWrapper. And object does not has __getattr__.

>>> object.__getattr__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'object' has no attribute '__getattr__'. Did you mean: '__setattr__'?

So this is incorrect without a doult, instead we should use getattr here, and to prevent infinite recursion, should add a check before calling getattr or hasattr in __getattr__.

---

All the cases will only happen when use in a wrong way, "accessing attributes without calling fit", so i just added some more intuitive error message comparing the old error message AttributeError: 'super' object has no attribute '__getattr__' .