scDelete logs IllegalStateException if SmartConnector is called when stopped

[Sophietje]

ERROR RestKnowledgeBaseManager - Encountered exception while stopping knowledge base
java.lang.IllegalStateException: The SmartConnector cannot be used anymore once it is stopped
        at eu.knowledge.engine.smartconnector.impl.SmartConnectorImpl.checkStopped(SmartConnectorImpl.java:475)
        at eu.knowledge.engine.smartconnector.impl.SmartConnectorImpl.stop(SmartConnectorImpl.java:459)
        at eu.knowledge.engine.rest.api.impl.RestKnowledgeBase.stop(RestKnowledgeBase.java:740)
        at eu.knowledge.engine.rest.api.impl.RestKnowledgeBaseManager.deleteKB(RestKnowledgeBaseManager.java:122)
        at eu.knowledge.engine.rest.api.impl.SmartConnectorLifeCycleApiServiceImpl.scDelete(SmartConnectorLifeCycleApiServiceImpl.java:194)
        at jdk.internal.reflect.GeneratedMethodAccessor37.invoke(Unknown Source)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:568)
        at org.glassfish.jersey.server.model.internal.ResourceMethodInvocationHandlerFactory.lambda$static$0(ResourceMethodInvocationHandlerFactory.java:52)
        at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher$1.run(AbstractJavaResourceMethodDispatcher.java:146)
        at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.invoke(AbstractJavaResourceMethodDispatcher.java:189)
        at org.glassfish.jersey.server.model.internal.JavaResourceMethodDispatcherProvider$VoidOutInvoker.doDispatch(JavaResourceMethodDispatcherProvider.java:159)
        at org.glassfish.jersey.server.model.internal.AbstractJavaResourceMethodDispatcher.dispatch(AbstractJavaResourceMethodDispatcher.java:93)
        at org.glassfish.jersey.server.model.ResourceMethodInvoker.invoke(ResourceMethodInvoker.java:478)
        at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:400)
        at org.glassfish.jersey.server.model.ResourceMethodInvoker.apply(ResourceMethodInvoker.java:81)
        at org.glassfish.jersey.server.ServerRuntime$1.run(ServerRuntime.java:261)
        at org.glassfish.jersey.internal.Errors$1.call(Errors.java:248)
        at org.glassfish.jersey.internal.Errors$1.call(Errors.java:244)
        at org.glassfish.jersey.internal.Errors.process(Errors.java:292)
        at org.glassfish.jersey.internal.Errors.process(Errors.java:274)
        at org.glassfish.jersey.internal.Errors.process(Errors.java:244)
        at org.glassfish.jersey.process.internal.RequestScope.runInScope(RequestScope.java:265)
        at org.glassfish.jersey.server.ServerRuntime.process(ServerRuntime.java:240)
        at org.glassfish.jersey.server.ApplicationHandler.handle(ApplicationHandler.java:697)
        at org.glassfish.jersey.servlet.WebComponent.serviceImpl(WebComponent.java:394)
        at org.glassfish.jersey.servlet.WebComponent.service(WebComponent.java:346)
        at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:357)
        at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:311)
        at org.glassfish.jersey.servlet.ServletContainer.service(ServletContainer.java:205)
        at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:764)
        at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:529)
        at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:221)
        at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1381)
        at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:176)
        at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:484)
        at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:174)
        at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1303)
        at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:129)
        at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:122)
        at org.eclipse.jetty.server.Server.handle(Server.java:563)
        at org.eclipse.jetty.server.HttpChannel.lambda$handle$0(HttpChannel.java:505)
        at org.eclipse.jetty.server.HttpChannel.dispatch(HttpChannel.java:762)
        at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:497)
        at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:282)
        at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:314)
        at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:100)
        at org.eclipse.jetty.io.SelectableChannelEndPoint$1.run(SelectableChannelEndPoint.java:53)
        at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:969)
        at org.eclipse.jetty.util.thread.QueuedThreadPool$Runner.doRunJob(QueuedThreadPool.java:1194)
        at org.eclipse.jetty.util.thread.QueuedThreadPool$Runner.run(QueuedThreadPool.java:1149)
        at java.base/java.lang.Thread.run(Thread.java:833)

We should improve the error handling here and not print the exception. See also the related issue #456