dispatch: ignore failure to flush ui When the pager dies, we get a `SIGPIPE`. That causes `error.SignalInterrupt` to be raised ` (from `ui._catchterm()`). Any further writes or flushes will cause further `SIGPIPE`s and furhter `error.SignalInterrupt`. If we write or flush outside of the try/except that handle `KeyboardInterrupt` (which `error.SignalInterrupt` is a subclass of), then control will escape from the `dispatch` module. Let's fix that by ignoring errors from flushing the ui. I would have rather fixed this by restoring the stdout and stderr streams when the pager dies, but it gets complicated because of multiple ui instances (ui/lui) and different pager setups between regular hg and chg. This changes a test in `test-pager.t`, but I don't understand why. I would have thought that all the output from the command should have gone to the broken pager. Differential Revision: https://phab.mercurial-scm.org/D11627