Offer a delayed Cancel while a tvOS channel loads, and fill the blank state

Without a cached header the channel showed only a spinner, and when the
channel request failed while the videos request succeeded (no cache to
build the channel from) it rendered nothing at all; both left the tvOS
remote with nothing to focus. Cancel stops at the error view and its
Retry, since a sidebar channel is a tab root that cannot be popped; the
blank state now shows the load error with Retry on every platform.

Yattee/Localizable.xcstrings

@@ -1745,6 +1745,17 @@
         }
       }
     },
+    "common.loadingStopped" : {
+      "comment" : "Shown in place of content after the user cancels a slow load on tvOS",
+      "localizations" : {
+        "en" : {
+          "stringUnit" : {
+            "state" : "translated",
+            "value" : "Loading was stopped."
+          }
+        }
+      }
+    },
     "common.loadMore" : {
       "localizations" : {
         "en" : {

Yattee/Views/Channel/ChannelView.swift

@@ -189,9 +189,21 @@ struct ChannelView: View {
                     loadingContent(cachedHeader)
                 } else if isLoading {
                     ProgressView()
+                        #if os(tvOS)
+                        // As a sidebar channel this is a tab root that cannot
+                        // be popped, so Cancel lands on the error and its Retry.
+                        .tvStalledLoadCancel {
+                            isLoading = false
+                            errorMessage = String(localized: "common.loadingStopped")
+                        }
+                        #endif
                         .frame(maxWidth: .infinity, maxHeight: .infinity)
                 } else if let error = errorMessage {
                     errorView(error)
+                } else {
+                    // The channel request failed while the videos one did not,
+                    // with no cached header to build the channel from.
+                    errorView(String(localized: "channelView.loadError"))
                 }
             }
         }