Improve UI error label, when server fails on loading a script config

[bugy]

When the server fails to load a script config (broken JSON, missing include dependency or anything else), then the client shows an error:

Failed to load script info. Try to reload the page. Error message:
Failed to connect to the server

The message is confusing. The server connection is there, but there is an internal server error and this information should be displayed (so an admin will have to check server logs).