Do more in Client.updateServers

[yiyin]

It should check for the pair of servers the client is working with.

For NLP server, if the current server is down, and another server of the same dataset can be found, it can freely switch to the other.

For NeuroArch Server, if the current server is down and another server of the same dataset can be found, we should use the popup interface in NeuroMynerva to ask if it is ok to switch (losing current work states) or wait.

In both cases, if no other servers can be found, a warning message should be displayed. Same when reconnection is made.