wip better message for handleEvent failure

Author: gnprice

assets/l10n/app_en.arb

@@ -172,6 +172,19 @@
       "error": {"type": "String", "example": "Invalid format"}
     }
   },
+  "errorHandlingEventTitle": "Error handling a Zulip event. Retrying connection…",
+  "@errorHandlingEventTitle": {
+    "description": "Error title on failing to handle a Zulip server event."
+  },
+  "errorHandlingEventDetails": "Error handling a Zulip event from {serverUrl}; will retry.\n\nError: {error}\n\nEvent: {event}",
+  "@errorHandlingEventDetails": {
+    "description": "Error details on failing to handle a Zulip server event.",
+    "placeholders": {
+      "serverUrl": {"type":  "String", "example":  "https://chat.example.com"},
+      "error": {"type": "String", "example": "Unexpected null value"},
+      "event": {"type": "String", "example": "UpdateMessageEvent(id: 123, messageIds: [2345, 3456], newTopic: 'dinner')"}
+    }
+  },
   "errorSharingFailed": "Sharing failed",
   "@errorSharingFailed": {
     "description": "Error message when sharing a message failed."

lib/model/store.dart

@@ -1050,7 +1050,10 @@ class UpdateMachine {
           assert(debugLog('BUG: Error handling an event: $cause\n' // TODO(log)
             '  event: $event\n'
             'Replacing event queue…'));
-          _reportToUserErrorConnectingToServer(e); // TODO better message
+          reportErrorToUserBriefly(
+            GlobalLocalizations.zulipLocalizations.errorHandlingEventTitle,
+            details: GlobalLocalizations.zulipLocalizations.errorHandlingEventDetails(
+              store.realmUrl.toString(), cause.toString(), event.toString()));
           // We can't just continue with the next event, because our state
           // may be garbled due to failing to apply this one (and worse,
           // any invariants that were left in a broken state from where