Fixes missed variable in compatibility layer which was causing importing subprocess to fail in some cases.
Added called process error to __all__
This looks like it duplicates #2860 which is on 1.3.x but has not been merged yet.
This does only a part of what #2860, does, but that issue will work as well. Feel free to close as soon as the other one is merged.
Closing because #2860 has been merged into 1.3.x which will make it into master sometime soon.