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>