rust-status: properly translate OSError to Python
This is probably never going to be called after the next few patches, but we
might as well make sure this is done correctly for the future rewrite.
Differential Revision: https://phab.mercurial-scm.org/D9319
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL