rpc-error in keepalive rpc-reply shouldn't bounce the session

If the keepalive rpc-reply has errors, it means the other end of the session is still available, hence the session shouldn't be bounced.

Current implementation for keepalive only checks for results but not for errors: https://github.com/opendaylight/netconf/blob/master/netconf/sal-netconf-connector/src/main/java/org/opendaylight/netconf/sal/connect/netconf/sal/KeepaliveSalFacade.java#L207

Implementation for other rpcs is ok, and doesn't even check the content of the result, assuming that as long as you have a result, the rpc reached the device: https://github.com/opendaylight/netconf/blob/master/netconf/sal-netconf-connector/src/main/java/org/opendaylight/netconf/sal/connect/netconf/sal/KeepaliveSalFacade.java#L228