Show an error message if starting the language server fails

[NiklasRentzCAU]

Whenever starting the language server from the JAR does not work, this error is not handled and not shown to the user. The user should be notified that starting up the language server failed and the reason for that.

One issue that is currently reported often is that the extension does not work properly with the message command 'klighd-vscode.diagram.open' not found when opening the diagram. This is most likely caused by a Java version lower than Java 17 that is used by the java command on the user's system, causing the LS startup to fail. If such an error is forwarded, the user could solve the problem by themselves.