#3566: Clean up handling of remote server disconnects.

#3566: Clean up handling of remote server disconnects.

This changeset does two things: introduces a new RemoteDisconnected exception
(that subclasses ConnectionResetError and BadStatusLine) so that a remote
server disconnection can be detected by client code (and provides a better
error message for debugging purposes), and ensures that the client socket is
closed if a ConnectionError happens, so that the automatic re-connection code
can work if the application handles the error and continues on.

Tests are added that confirm that a connection is re-used or not re-used
as appropriate to the various combinations of protocol version and headers.

Patch by Martin Panter, reviewed by Demian Brecht.  (Tweaked only slightly by
me.)