Do not start faulty coronas

It just floods the output with tons of things failing non-fatally and it
makes things hard to figure out.

shell/main.cpp

@@ -165,6 +165,10 @@ int main(int argc, char *argv[])
 
         ShellCorona *corona = new ShellCorona(&app);
         corona->setShell(cliOptions.value(shellPluginOption));
+        if (!corona->kPackage().isValid()) {
+            qCritical() << "starting invalid corona" << corona->shell();
+            return 1;
+        }
 
 #ifdef WITH_KUSERFEEDBACKCORE
         auto userFeedback = new UserFeedback(corona, &app);

shell/standaloneappcorona.cpp

@@ -47,6 +47,9 @@ StandaloneAppCorona::StandaloneAppCorona(const QString &coronaPlugin, QObject *p
     KPackage::Package package = KPackage::PackageLoader::self()->loadPackage(QStringLiteral("Plasma/Shell"));
     package.setPath(m_coronaPlugin);
     package.setAllowExternalPaths(true);
+    if (!package.isValid()) {
+        qCritical() << "starting invalid corona" << m_coronaPlugin;
+    }
     setKPackage(package);
 
     Plasma::Theme theme;