Fixed #4444 - Tidying up 'broken pipe' errors in runserver logs

I have posted about this pull request on the django-developers mailing list at https://groups.google.com/forum/#!topic/django-developers/_oC_PAxxnQI . I did this because there is a very old existing wontfix ticket ([#4444](https://code.djangoproject.com/ticket/4444)) on this topic and so am hopefully following the contribution guidelines.

Ticket #4444 was opened in May 2007, and marked wontfix in September 2007 saying "The best we could is to have a more explicit error message." - hopefully this might count as such an error message :smile:

This pull request builds upon a previous patch posted to the django-developers list and outputs "Broken pipe" instead of a traceback for such an error. I get these frequently (e.g. hit refresh before your previous page has finished loading to sometimes get it).

I hope this can be considered for inclusion, as I think it tidies up a common issue with runserver output that will especially confuse people new to Django. Do let me know if I've gone about fixing it in the wrong way, or if I should do something differently.
Thanks for looking at this :) I see why you would say that, sure - although I might as well note that the change, made only last month (hence why this PR doesn't currently match it), is the only change to that function since it became part of the standard library in 2006, and is a small potential security issue that doesn't apply to a development server in any case, as that's explicitly listed as being insecure.

I don't necessarily think you'd have to keep changes in sync from upstream, as I don't see there would be any changes to this function other than other tiny security issues that don't matter to a development server, but I can see you wouldn't want to be in that potential situation anyway.

The underlying issue/cause is that there doesn't otherwise appear to be a way to pass in a different `ServerHandler` (which is what is actually wanted), and I doubt the standard library will gain that functionality. With just the `WSGIServer` change, it would catch some of the pipe exceptions, but not all, sadly. Hohum.