Integrate MAS with Synapse

mas/share/templates/pages/error.html

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+{#
+Copyright 2024, 2025 New Vector Ltd.
+Copyright 2021-2024 The Matrix.org Foundation C.I.C.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+-#}
+
+{# Sometimes we don't have the language set, so we default to english #}
+{% set lang = lang or "en" %}
+
+{% extends "base.html" %}
+
+{% block content %}
+  <main class="flex flex-col gap-6">
+    <header class="page-heading">
+      <div class="icon invalid">
+        {{ icon.error_solid() }}
+      </div>
+
+      <div class="header">
+        <h1 class="title">{{ _("error.unexpected") }}</h1>
+        {% if code %}
+          <p class="text font-semibold font-mono">
+            {{ code }}
+          </p>
+        {% endif %}
+        {% if description %}
+          <p class="text">
+            {{ description }}
+          </p>
+        {% endif %}
+      </div>
+    </header>
+
+    {% if details %}
+      <hr />
+      {# caution: do not introduce whitespace between <pre> and <code> #}
+      <pre><code class="font-mono whitespace-pre-wrap break-all">{{ details }}</code></pre>
+    {% endif %}
+  </main>
+{% endblock %}