There are plenty of other "bad" exception classes apart from CalledProcessError, including TimeoutExpired in the same file.  In fact I suspect this is true of the majority of the exception classes in the stdlib which override __init__.  So I am not sure how much good it would do to fix just one example.

Python 3.x's Pool wraps bad exception instances in a MaybeEncodingError class which at least lets you see a stringification of the original exception.  I am not sure whether you would want to see a backport of this.  Even though 2.7 is in bug fix mode, I think a backport would still be appropriate since it stops a pickling error from killing a worker process, causing a hang.