In case server failed to start, do not throw error on remove.

This happens when quitting an external app which would tell the dryad tree
to remove everything; even things that were never successfully added.

src/dryads/SCServer.js

@@ -39,7 +39,9 @@ export default class SCServer extends Dryad {
   remove() {
     return {
       run: (context) => {
-        return context.scserver.quit();
+        if (context.scserver) {
+          return context.scserver.quit();
+        }
       }
     };
   }