install symfony demo with installer

app/templates/bundles/TwigBundle/Exception/error.html.twig

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+{#
+    This template is used to render any error different from 403, 404 and 500.
+
+    This is the simplest way to customize error pages in Symfony applications.
+    In case you need it, you can also hook into the internal exception handling
+    made by Symfony. This allows you to perform advanced tasks and even recover
+    your application from some errors.
+    See https://symfony.com/doc/current/controller/error_pages.html
+#}
+
+{% extends 'base.html.twig' %}
+
+{% block body_id 'error' %}
+
+{% block main %}
+    <h1 class="text-danger">{{ 'http_error.name'|trans({ '%status_code%': status_code }) }}</h1>
+
+    <p class="lead">
+        {{ 'http_error.description'|trans({ '%status_code%': status_code }) }}
+    </p>
+    <p>
+        {{ 'http_error.suggestion'|trans({ '%url%': path('blog_index') })|raw }}
+    </p>
+{% endblock %}
+
+{% block sidebar %}
+    {{ parent() }}
+
+    {{ show_source_code(_self) }}
+{% endblock %}

app/templates/bundles/TwigBundle/Exception/error403.html.twig

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+{#
+    This template is used to render errors of type HTTP 403 (Forbidden)
+
+    This is the simplest way to customize error pages in Symfony applications.
+    In case you need it, you can also hook into the internal exception handling
+    made by Symfony. This allows you to perform advanced tasks and even recover
+    your application from some errors.
+    See https://symfony.com/doc/current/controller/error_pages.html
+#}
+
+{% extends 'base.html.twig' %}
+
+{% block body_id 'error' %}
+
+{% block main %}
+    <h1 class="text-danger"><i class="fa fa-unlock-alt" aria-hidden="true"></i> {{ 'http_error.name'|trans({ '%status_code%': 403 }) }}</h1>
+
+    <p class="lead">
+        {{ 'http_error_403.description'|trans }}
+    </p>
+    <p>
+        {{ 'http_error_403.suggestion'|trans }}
+    </p>
+{% endblock %}
+
+{% block sidebar %}
+    {{ parent() }}
+
+    {{ show_source_code(_self) }}
+{% endblock %}

app/templates/bundles/TwigBundle/Exception/error404.html.twig

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+{#
+    This template is used to render errors of type HTTP 404 (Not Found)
+
+    This is the simplest way to customize error pages in Symfony applications.
+    In case you need it, you can also hook into the internal exception handling
+    made by Symfony. This allows you to perform advanced tasks and even recover
+    your application from some errors.
+    See https://symfony.com/doc/current/controller/error_pages.html
+#}
+
+{% extends 'base.html.twig' %}
+
+{% block body_id 'error' %}
+
+{% block main %}
+    <h1 class="text-danger">{{ 'http_error.name'|trans({ '%status_code%': 404 }) }}</h1>
+
+    <p class="lead">
+        {{ 'http_error_404.description'|trans }}
+    </p>
+    <p>
+        {{ 'http_error_404.suggestion'|trans({ '%url%': path('blog_index') })|raw }}
+    </p>
+{% endblock %}
+
+{% block sidebar %}
+    {{ parent() }}
+
+    {{ show_source_code(_self) }}
+{% endblock %}

app/templates/bundles/TwigBundle/Exception/error500.html.twig

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+{#
+    This template is used to render errors of type HTTP 500 (Internal Server Error)
+
+    This is the simplest way to customize error pages in Symfony applications.
+    In case you need it, you can also hook into the internal exception handling
+    made by Symfony. This allows you to perform advanced tasks and even recover
+    your application from some errors.
+    See https://symfony.com/doc/current/controller/error_pages.html
+#}
+
+{% extends 'base.html.twig' %}
+
+{% block stylesheets %}
+    {{ parent() }}
+    <link rel="stylesheet" href="/build/css/app.4aa95248.css">
+{% endblock %}
+
+{% block body_id 'error' %}
+
+{% block main %}
+    <h1 class="text-danger">{{ 'http_error.name'|trans({ '%status_code%': 500 }) }}</h1>
+
+    <p class="lead">
+        {{ 'http_error_500.description'|trans }}
+    </p>
+    <p>
+        {{ 'http_error_500.suggestion'|trans({ '%url%': path('blog_index') })|raw }}
+    </p>
+{% endblock %}
+
+{% block sidebar %}
+    {{ parent() }}
+
+    {{ show_source_code(_self) }}
+{% endblock %}