Add indication when xi-core is not available

[NoraCodes]

trafficstars

Currently, if xi-core isn't available, there is a GTK+ CRITICAL-level message, but no user-facing message.

(xi-gtk:8793): CRITICAL **: CoreConnection.vala:268: Failed to execute child process “xi-core” (No such file or directory)

It would be great if a dialog would pop up and then the editor would exit; as it is I was very confused as to why it wasn't working.

[eyelash]

In the future I will provide binary packages and they will make sure xi-core is correctly installed.

[NoraCodes]

Still, especially on Arch systems, I think it's a good idea to have a more explicitly specified behavior when xi-core isn't available; even if the binary is there, $PATH could be messed up, for instance.

[rafaelmardojai]

Would be nice to have screens for these kind of errors, something like this.