Show a clearer error message if the initial UI load fails

This should only ever be possible on the initial page load, if no
network connection is available, since after that point the service
worker will always have the UI cached.

Author: pimterry

src/index.ts

@@ -235,6 +235,30 @@ if (!amMainInstance) {
                 contents.reload();
             });
         });
+
+        contents.on('did-fail-load', (
+            _event,
+            code,
+            description,
+            url,
+            isMainFrame
+        ) => {
+            if (!isMainFrame) return; // Just in case
+
+            const { protocol, host, pathname } = new URL(url);
+            const baseURL = `${protocol}//${host}${pathname}`;
+
+            showErrorAlert(
+                "UI load failed",
+                `The HTTP Toolkit UI could not be loaded from\n${baseURL}.` +
+                "\n\n" +
+                `${description} (${code})`
+            );
+
+            setTimeout(() => {
+                contents.reload();
+            }, 2000);
+        });
     });
 
     function checkForUnsafeNavigation(url: URL) {