ui: ignore EIO in write_err
Hgs signal handler will catch the signal for example if the terminal hg is
running in is closed. That will make it try to warn that it was 'killed', but
that might fail with EIO and cause hg to exit with an unhandled exception.
Normally nobody cares, but system error handlers such as Fedoras abrt will
notice and report https://bugzilla.redhat.com/show_bug.cgi?id=596594 .
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://mercurial.selenic.com/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.