hook: ensure stderr is flushed when an exception is raised, for test stability
Windows has had issues with output order in test-ssh-proto-unbundle.t[1] since
it was created a few weeks ago. Each of the problems occurred when an exception
was thrown out of the hook.
Now the only thing blocking D2720 is the fact that the "abort: ..." lines on
stderr are totally AWOL. I have no idea where there are.
[1] https://buildbot.mercurial-scm.org/builders/Win7%20x86_64%20hg%20tests/builds/541/steps/run-tests.py%20%28python%202.7.13%29/logs/stdio
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