fire pixels when showing the error screen (#3462)

Task/Issue URL:
https://app.asana.com/0/392891325557410/1208511720659950/f
Tech Design URL:
CC:

**Description**:
Fires a pixel when the error screen is shown.

**Steps to test this PR**:
1. Open a tab, and use activity monitor to kill that process. The
"webpage has crashed" error screen should appear and the pixel should be
fired.
2. Open a nonsense url like https://nonsense.url - The 'unable to load'
page should appear and the pixel should be displayed
3. Switch between other tabs and the error pages and pixels should be
fired when the user switches tabs.

**Definition of Done**:

* [ ] Does this PR satisfy our [Definition of
Done](https://app.asana.com/0/1202500774821704/1207634633537039/f)?

---
###### Internal references:
[Pull Request Review
Checklist](https://app.asana.com/0/1202500774821704/1203764234894239/f)
[Software Engineering
Expectations](https://app.asana.com/0/59792373528535/199064865822552)
[Technical Design
Template](https://app.asana.com/0/59792373528535/184709971311943)
[Pull Request
Documentation](https://app.asana.com/0/1202500774821704/1204012835277482/f)

DuckDuckGo/Statistics/GeneralPixel.swift

@@ -328,6 +328,7 @@ enum GeneralPixel: PixelKitEventV2 {
     case adAttributionLogicWrongVendorOnFailedCompilation
 
     case webKitDidTerminate
+    case userViewedWebKitTerminationErrorPage
 
     case removedInvalidBookmarkManagedObjects
 
@@ -424,6 +425,10 @@ enum GeneralPixel: PixelKitEventV2 {
 
     case compilationFailed
 
+    // MARK: error page shown
+    case errorPageShownOther
+    case errorPageShownWebkitTermination
+
     var name: String {
         switch self {
 
@@ -916,6 +921,8 @@ enum GeneralPixel: PixelKitEventV2 {
 
         case .webKitDidTerminate:
             return "webkit_did_terminate"
+        case .userViewedWebKitTerminationErrorPage:
+            return "webkit-termination-error-page-viewed"
 
         case .removedInvalidBookmarkManagedObjects:
             return "removed_invalid_bookmark_managed_objects"
@@ -1037,6 +1044,9 @@ enum GeneralPixel: PixelKitEventV2 {
         case .bookmarksSortByName: return "m_mac_sort_bookmarks_by_name"
         case .bookmarksSearchExecuted: return "m_mac_search_bookmarks_executed"
         case .bookmarksSearchResultClicked: return "m_mac_search_result_clicked"
+
+        case .errorPageShownOther: return "m_mac_errorpageshown_other"
+        case .errorPageShownWebkitTermination: return "m_mac_errorpageshown_webkittermination"
         }
     }
 

DuckDuckGo/Tab/Model/Tab.swift

@@ -1259,6 +1259,8 @@ extension Tab/*: NavigationResponder*/ { // to be moved to Tab+Navigation.swift
 
     @MainActor
     func webContentProcessDidTerminate(with reason: WKProcessTerminationReason?) {
+        guard (error?.code.rawValue ?? WKError.Code.unknown.rawValue) != WKError.Code.webContentProcessTerminated.rawValue else { return }
+
         let error = WKError(.webContentProcessTerminated, userInfo: [
             WKProcessTerminationReason.userInfoKey: reason?.rawValue ?? -1,
             NSLocalizedDescriptionKey: UserText.webProcessCrashPageMessage

DuckDuckGo/TabBar/ViewModel/TabCollectionViewModel.swift

@@ -130,6 +130,7 @@ final class TabCollectionViewModel: NSObject {
 
         subscribeToTabs()
         subscribeToPinnedTabsManager()
+        subscribeToSelectedTab()
 
         if tabCollection.tabs.isEmpty {
             appendNewTab(with: homePage)
@@ -153,6 +154,32 @@ final class TabCollectionViewModel: NSObject {
                   burnerMode: burnerMode)
     }
 
+    var selectedTabCancellable: AnyCancellable?
+    private func subscribeToSelectedTab() {
+        selectedTabCancellable = $selectedTabViewModel
+            .compactMap { $0 }
+            .sink { [weak self] model in
+                self?.subscribeToTabError(model)
+            }
+    }
+
+    var selectedTabErrorCancellable: AnyCancellable?
+    private func subscribeToTabError(_ model: TabViewModel) {
+        selectedTabErrorCancellable = model.tab.$error
+            .compactMap { $0 }
+            .sink { [weak self] error in
+                self?.fireErrorPageShownPixel(error)
+        }
+    }
+
+    private func fireErrorPageShownPixel(_ error: WKError) {
+        if error.code == WKError.Code.webContentProcessTerminated {
+            PixelKit.fire(GeneralPixel.errorPageShownWebkitTermination)
+        } else {
+            PixelKit.fire(GeneralPixel.errorPageShownOther)
+        }
+    }
+
     func setUpLazyLoadingIfNeeded() {
         guard !isTabLazyLoadingRequested else {
             Logger.tabLazyLoading.debug("Lazy loading already requested in this session, skipping.")