Language server fails to start when a meta-model on the meta-model search path contains a reference to an unknown type

[larsk21]

Opening a NeoJoin query in VS Code with a meta-model on the meta-model search path, which contains a reference to an unknown type, leads to the following error:

Error - 4:12:37 PM] Server initialization failed.
  Message: Internal error.
  Code: -32603
[Error - 4:12:37 PM] NeoJoin Language Server client: couldn't create connection to server.
  Message: Internal error.
  Code: -32603

The error is caused by an assertion in the SourceModelInferrer:

at tools.vitruv.neojoin.jvmmodel.SourceModelInferrer.getReferenceType(SourceModelInferrer.java:187)

In my case, it was caused by a reference to an element of type EObject:

<eStructuralFeatures xsi:type="ecore:EReference" name="referencedComponents" upperBound="-1"
        eType="ecore:EClass http://www.eclipse.org/emf/2002/Ecore#//EObject"/>

We could consider loading the Ecore meta-model by default. In any case, we should display a warning in the IDE and avoid crashing the server upon any problems during meta-model loading.