[Python-Dev] Adding NetworkIOError for bug 1706815

[Gregory P. Smith]

In response to bug 1706815 and seeing messy code to catch errors in
network apps I've implemented most of the ideas in the bug and added a
NetworkIOError exception (child of IOError).  With this, socket.error
would now inherit from NetworkIOError instead of being its own thing
(the old one didn't even declare a parent!).

Complete patch attached to the bug.  All unit tests pass.
Documentation updates included.

 http://sourceforge.net/tracker/index.php?func=detail&aid=1706816&group_id=5470&atid=105470

Any thoughts?  I'm happy with it and would like to commit it if folks
agree.