Display error if duplicate install detected

Detection may not be the most reliable if one of the duplicates is not
seeing updates and happens to install first. I'll keep pondering a more
reliable way to do this. The only ideas that come to mind are a little
too hacky and unsafe for my current standards.

web/js/VHS.core.js

@@ -171,6 +171,11 @@ var helpDOM = app.VHSHelp;
 if (!app.VHSHelp) {
     helpDOM = document.createElement("div");
     app.VHSHelp = helpDOM
+} else {
+    app.extensionManager.dialog
+      .showErrorDialog('Please check your custom_nodes directory and manually remove the duplicate.',
+                       { title: 'Duplicate VHS install detected' })
+    throw new Error('Duplicate VHS install detected. Check your custom_nodes directory')
 }
 function initHelpDOM() {
     let parentDOM = document.createElement("div");