CLI/Language Server error handling on non-existent/erroneous models

[NiklasRentzCAU]

There are a few usability issues with the current CLI/Language Server handling:

• The CLI should show a persistent error message if the model that should be shown does not exist. Currently the view just shows a "No model in resource" diagram, but no error message indicating that the model just does not exist.
• The CLI/Language Server should display a persistent error message if the model that should be shown is erroneous. For example, showing a OSGi Model with the OSGiViz Language Server in for a .osgi file that has syntax errors, a good error message should be shown that the model could not load and what the reason for that was. Currently only this non-speaking error message is printed:

Error during request: 
java.lang.NullPointerException: Cannot invoke "org.eclipse.sprotty.SModelRoot.getType()" because "server.currentRoot" is null
	at de.cau.cs.kieler.klighd.lsp.KGraphDiagramUpdater.lambda$updateDiagram$5(KGraphDiagramUpdater.java:212)
	at de.cau.cs.kieler.klighd.lsp.KGraphLanguageServerExtension.lambda$doRead$21(KGraphLanguageServerExtension.java:683)
	at org.eclipse.xtext.ide.server.concurrent.ReadRequest.lambda$1(ReadRequest.java:66)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
	at java.base/java.lang.Thread.run(Thread.java:840)

This issue might be tackled in multiple steps for each sub-issue