store: Add details to error on no PerAccountStoreWidget

This draws from MediaQuery.of and debugCheckHasMediaQuery.

Prompted by this question:
  https://chat.zulip.org/#narrow/channel/516-mobile-dev-help/topic/No.20PerAccountStoreWidget.20ancestor/near/2008484

lib/widgets/store.dart

@@ -164,7 +164,7 @@ class PerAccountStoreWidget extends StatefulWidget {
   ///  * [InheritedNotifier], which provides the "dependency" mechanism.
   static PerAccountStore of(BuildContext context) {
     final widget = context.dependOnInheritedWidgetOfExactType<_PerAccountStoreInheritedWidget>();
-    assert(widget != null, 'No PerAccountStoreWidget ancestor');
+    assert(_debugCheckFound(context, widget));
     return widget!.store;
   }
 
@@ -183,12 +183,29 @@ class PerAccountStoreWidget extends StatefulWidget {
   /// Like [of], the cost of this method is O(1) with a small constant factor.
   static int accountIdOf(BuildContext context) {
     final element = context.getElementForInheritedWidgetOfExactType<_PerAccountStoreInheritedWidget>();
-    assert(element != null, 'No PerAccountStoreWidget ancestor');
+    assert(_debugCheckFound(context, element));
     final widget = element!.findAncestorWidgetOfExactType<PerAccountStoreWidget>();
     assert(widget != null);
     return widget!.accountId;
   }
 
+  static bool _debugCheckFound(BuildContext context, Object? ancestor) {
+    // Compare [debugCheckHasMediaQuery], and its caller [MediaQuery.of].
+    assert(() {
+      if (ancestor != null) return true;
+      throw FlutterError.fromParts([
+        ErrorSummary('No PerAccountStoreWidget ancestor found.'),
+        ErrorDescription('${context.widget.runtimeType} widgets require a PerAccountStoreWidget ancestor.'),
+        context.describeWidget('The specific widget that could not find a PerAccountStoreWidget ancestor was'),
+        context.describeOwnershipChain('The ownership chain for the affected widget is'),
+        ErrorHint('For a new page in the app, consider MaterialAccountWidgetRoute '
+          'or AccountPageRouteBuilder.'),
+        ErrorHint('In tests, consider TestZulipApp with its accountId field.'),
+      ]);
+    }());
+    return true;
+  }
+
   /// Whether there is a relevant account specified for this widget.
   static bool debugExistsOf(BuildContext context) {
     return context.getElementForInheritedWidgetOfExactType<_PerAccountStoreInheritedWidget>() != null;

test/widgets/store_test.dart

@@ -95,6 +95,24 @@ void main() {
     tester.widget(find.text('found store, account: ${eg.selfAccount.id}'));
   });
 
+  testWidgets('PerAccountStoreWidget.of detailed error', (tester) async {
+    addTearDown(testBinding.reset);
+    await tester.pumpWidget(
+      Directionality(
+        textDirection: TextDirection.ltr,
+        child: GlobalStoreWidget(
+          // no PerAccountStoreWidget
+          child: Builder(
+            builder: (context) {
+              final store = PerAccountStoreWidget.of(context);
+              return Text('found store, account: ${store.accountId}');
+            }))));
+    await tester.pump();
+    check(tester.takeException())
+      .has((x) => x.toString(), 'toString') // TODO(checks): what's a good convention for this?
+      .contains('consider MaterialAccountWidgetRoute');
+  });
+
   testWidgets('PerAccountStoreWidget immediate data after first loaded', (tester) async {
     await testBinding.globalStore.add(eg.selfAccount, eg.initialSnapshot());
     addTearDown(testBinding.reset);