Update error handling and add more comments

Modules/ZoomIn/Sources/ViewController.swift

@@ -4,8 +4,24 @@ import SafariServices.SFSafariApplication
 class ViewController: NSViewController {
     @IBAction func openSafariExtensionPreferences(_ sender: AnyObject?) {
         SFSafariApplication.showPreferencesForExtension(withIdentifier: "com.nothingmagical.zoom-in.extension") { error in
-            if let error = error {
-                print("Dang: \(error)")
+            guard let error = error else {
+                return
+            }
+
+            // This callback is called on a background thread. Switch to the main thread.
+            DispatchQueue.main.async { [weak self] in
+                // Setup the error
+                let alert = NSAlert(error: error)
+
+                // Add some more information
+                alert.informativeText = "Failed to open Safari preferences."
+
+                // Show it from our window (or fallback to just showing on its own)
+                if let window = self?.view.window {
+                    alert.beginSheetModal(for: window, completionHandler: nil)
+                } else {
+                    alert.runModal()
+                }
             }
         }
     }

Modules/ZoomIn/Sources/WindowController.swift

@@ -3,10 +3,13 @@ import AppKit
 final class WindowController: NSWindowController, NSWindowDelegate {
     override func windowDidLoad() {
         super.windowDidLoad()
+
+        // Center the window on the screen when it loads
         window?.center()
     }
 
     func windowWillClose(_ notification: Notification) {
+        // When the window closes, quit the app
         NSApp.terminate(notification)
     }
 }

Modules/ZoomInExtension/Resources/script.js

@@ -1,4 +1,8 @@
+// Wait for the page to load
 document.addEventListener("DOMContentLoaded", function(event) {
+    // Get the URL from Zoom’s page
     const url = document.getElementById('scheme_url')['value'];
+
+    // Tell the native extension about the URL
     safari.extension.dispatchMessage('open', { url: url });
 });