Improve error message while setting error twice. (#54464)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/54464
In case where we accidentaly set an error twice on a Future, we get a
cryptic error like this:
```
Exception in thread pool task: !completed() INTERNAL ASSERT FAILED at "aten/src/ATen/core/ivalue_inl.h":534, please report a bug to PyTorch.
```
This PR, updates the error message to include some additional information about
what the previous error was.
ghstack-source-id: 125039478
Test Plan:
1) unit test
2) waitforbuildbot
Reviewed By: swolchok
Differential Revision: D27249758
fbshipit-source-id: 517cf3837fb7b7821312e101e8813844c188f372
