client rpc: Report proper error for keepalive disconnections

client rpc: Report proper error for keepalive disconnections

Whenever a connection was closed due to keepalive timeout, we would log
a warning but the interrupted API would return rather useless generic
error:

    internal error: received hangup / error event on socket

Let's report a proper keepalive timeout error and make sure it is
propagated to all pending APIs. The error should be better now:

    internal error: connection closed due to keepalive timeout

Based on an old patch from Martin Kletzander.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>