[caffe2][torch] correctly re-raise Manifold StorageException
Summary:
1) Manifold raises StorageException when it see's an error: https://fburl.com/diffusion/kit3me8a
2) torch re-raises exception: https://fburl.com/diffusion/zbw9wmpu
Issue here, that in StorageException first argument is bool canRetry while re-raising happens with first argument being str as in all Python exceptions.
Test Plan:
Existing tests should pass. +
```
In [1]: from manifold.clients.python import StorageException
In [2]: getattr(StorageException, "message", None)
Out[2]: <attribute 'message' of 'manifold.blobstore.blobstore.types.StorageException' objects>
In [3]: getattr(Exception, "message", None) is None
Out[3]: True
Reviewed By: haijunz
Differential Revision: D23195514
fbshipit-source-id: baa1667dbba4086db6ec93f009e400611ac9b938
