commandserver: handle IOError related to flushing of streams After dispatch, without chg we have handling of flushing of streams and exception handling related to it. The exception handling part is important because there can be exceptions when flushing fout or ferr. One such case is in `test-basic.t` which was failing on python3+chg without this patch as this handling was missing from chg. Failure can be seen at https://foss.heptapod.net/octobus/mercurial-devel/-/jobs/128399 Honestly I am not sure which one of `chgserver.py` or `commandserver.py` the change should go in. Differential Revision: https://phab.mercurial-scm.org/D9517

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