Make sure all our custom exceptions can be pickled so that if they
get thrown in a multiprocess subprocess, we don't crash & hang due
to multiprocessing being unable to pickle+unpickle the exception.
Details/examples can be seen in Python reports like:
https://bugs.python.org/issue13751
Change-Id: Iddf14d3952ad4e2867cfc71891d6b6559130df4b
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297382
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
