AsyncUDPSocket.handle_error

By overriding the default asyncore handle_error (which closes the
socket) with our own version, which logs what happened but tries to
proceed, we can get rid of a couple of try/except blocks. The resulting
churn is deindentation of the internal code.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>