Show errors when video loading fails

Closes #532

Tusker/Screens/Gallery/VideoGalleryContentViewController.swift

@@ -116,12 +116,45 @@ class VideoGalleryContentViewController: UIViewController, GalleryContentViewCon
             MainActor.runUnsafely {
                 if item.status == .readyToPlay {
                     self.container?.setGalleryContentLoading(false)
-                    statusObservation = nil
+                    self.statusObservation = nil
+                } else if item.status == .failed,
+                          let error = item.error {
+                    self.container?.setGalleryContentLoading(false)
+                    self.showErrorView(error)
+                    self.statusObservation = nil
                 }
             }
         })
     }
     
+    private func showErrorView(_ error: any Error) {
+        let image = UIImageView(image: UIImage(systemName: "exclamationmark.triangle.fill")!)
+        image.tintColor = .secondaryLabel
+        image.contentMode = .scaleAspectFit
+        
+        let label = UILabel()
+        label.text = "Error Loading"
+        label.font = .preferredFont(forTextStyle: .title1).withTraits(.traitBold)!
+        label.textColor = .secondaryLabel
+        label.adjustsFontForContentSizeCategory = true
+        
+        let stackView = UIStackView(arrangedSubviews: [
+            image,
+            label,
+        ])
+        stackView.translatesAutoresizingMaskIntoConstraints = false
+        stackView.axis = .vertical
+        stackView.alignment = .center
+        stackView.spacing = 8
+        view.addSubview(stackView)
+        NSLayoutConstraint.activate([
+            image.widthAnchor.constraint(equalToConstant: 64),
+            image.heightAnchor.constraint(equalToConstant: 64),
+            stackView.centerXAnchor.constraint(equalTo: view.centerXAnchor),
+            stackView.centerYAnchor.constraint(equalTo: view.centerYAnchor),
+        ])
+    }
+    
     @objc private func preferencesChanged() {
         if isGrayscale != Preferences.shared.grayscaleImages {
             let isPlaying = player.rate > 0