dispatch: don't change error status if flushing stdio fails
If we already have a non-zero exit code, I don't think we should
change it to 255 because we fail to flush stdio. This may not matter
yet, but it will matter when I make a killed pager result in exit code
250 (it's currently 255).
Differential Revision: https://phab.mercurial-scm.org/D11626