Add missing templates

templates/flash_msg.html

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+{% with messages = get_flashed_messages() %}
+  {% if messages %}
+    {% for message in messages %}
+      <div class="alert alert-success alert-dismissible" role="alert">
+        <button type="button" class="close" data-dismiss="alert" aria-label="Close">
+          <span aria-hidden="true">&times;</span>
+        </button>
+        {{ message }}
+      </div>
+    {% endfor %}
+  {% endif %}
+{% endwith %}

templates/show_error.html

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+{% extends "base.html" %}
+
+{% block style %}
+<link rel="stylesheet" href="{{url_for('static', filename='css/exception.css')}}" />
+{% endblock %}
+
+{% block content %}
+
+<div class="container my-2">
+<div class="row">
+<div class="col">
+
+<h1>Software error: {{ tb.exception_type }}</h1>
+<div>
+  <pre>{{ tb.exception }}</pre>
+</div>
+
+{% set body %}
+URL: {{ request.url }}
+
+{{ tb.plaintext | safe }}
+{% endset %}
+
+<p><a class="btn btn-primary btn-lg" role="button" href="https://github.com/EdwardBetts/osm-wikidata/issues/new?title={{ tb.exception | urlencode }}&body={{ body | urlencode }}">Submit as an issue on GitHub</a> (requires an account with GitHub)</p>
+
+<h2 class="traceback">Traceback <em>(most recent call last)</em></h2>
+{{ tb.render_summary(include_title=False) | safe }}
+
+<p>Error in function "{{ tb.frames[-1].function_name }}": {{ last_frame_args | pprint }}</p>
+<pre>{{ last_frame.locals | pprint }}</pre>
+
+</div>
+</div>
+</div>
+
+{% endblock %}