Fix exception chaining all over the codebase

[akihironitta]

I would like to suggest a small fix in the way that Python 3's exception chaining is used.

As described in this article, exception chaining can be used to make exceptions more user-friendly, and in that case, the syntax raise new_exception from old_exception needs to be used.

For example,

try:
    something_which_raises_TypeError
except TypeError as e:
    raise ValueError("a more user-friendly exception message.") from e

the above should be used instead of

try:
    something_which_raises_TypeError
except TypeError:
    raise ValueError("a more user-friendly exception message.")

The following files (or possibly more) can be modified according to this syntax:

• detectron2/detectron2/data/catalog.py

  Line 52 in f3ada7d

  except KeyError:

• detectron2/detectron2/config/config.py

  Line 141 in 439134d

  except AttributeError:

• detectron2/detectron2/data/transforms/augmentation.py

  Line 88 in f3ada7d

  except AttributeError:

• detectron2/detectron2/engine/hooks.py

  Line 334 in 439134d

  except Exception:

Let me know your thoughts on this!

[ppwwyyxx]

closed in #1945. thanks for the PR!