message: Remove failure message for disabled read receipts.

This removes the "Failed to load read receipts." prefix
in the read receipt overlay in case of organizations
that have disabled their read receipts.

Fixes: https://chat.zulip.org/#narrow/channel/9-issues/topic/Why.20the.20failure.20message.20on.20disabled.20read.20receipts.2E

web/src/read_receipts.ts

@@ -12,6 +12,7 @@ import * as loading from "./loading.ts";
 import * as message_store from "./message_store.ts";
 import * as modals from "./modals.ts";
 import * as people from "./people.ts";
+import {realm} from "./state_data.ts";
 import * as ui_report from "./ui_report.ts";
 import * as util from "./util.ts";
 
@@ -36,6 +37,16 @@ export function fetch_read_receipts(message_id: number): void {
         $("#read_receipts_modal .modal__content").addClass("compact");
         return;
     }
+    if (!realm.realm_enable_read_receipts) {
+        ui_report.error(
+            $t({
+                defaultMessage: "Read receipts are disabled for this organization.",
+            }),
+            undefined,
+            $("#read_receipts_modal #read_receipts_error"),
+        );
+        return;
+    }
 
     if (!has_initial_data) {
         loading.make_indicator($("#read_receipts_modal .loading_indicator"));