Improve err message when trying to connect from another client

[marimeireles]

Discussion: jupyter-xeus/xeus-sqlite#81

@martinRenou where would be a good place to add this function?
xconfiguration maybe?

[martinRenou]

Maybe yes? I am not sure what form this would take, but maybe a free function or something that tries to factorize a bit of this code that was duplicated in different kernels.

[JohanMabille]

A better option would be to add this message as a part of the xkernel::start method:

void start(const std::string& kernel_name, const std::string& config_file_name);
void start(const std::string& kernel_name);

That would be a backward incompatible change though.