Added custom error pages, that assists users with common problems.

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

@@ -0,0 +1,35 @@
+{% extends "bundles/TwigBundle/Exception/error.html.twig" %}
+
+{% block status_comment %}
+    Something bad happened internally.
+    <br>There is nothing you could do about this, except trying to reload the page.
+{% endblock %}
+
+{% block admin_info %}
+    {% if exception.class == "Doctrine\\DBAL\\Exception\\ConnectionException" %}
+        <i>Can not connect to database.</i> Try follwing things:
+        <ul>
+            <li>Check if the database server is running</li>
+            <li>Ensure that <code>DATABASE_URL</code> in <code>.env.local</code> is correct: database name, user and password must be correct.</li>
+            <li>Ensure that the database user has access to the database.</li>
+        </ul>
+    {% elseif exception.class == "Twig\\Error\\RuntimeError" and 'manifest.json' in exception.message %}
+        <i>Can not load frontend assets.</i> Try following things:
+        <ul>
+            <li>Run <kbd>yarn install</kbd> and <kbd>yarn build</kbd> in Part-DB folder.</li>
+            <li>Run <kbd>php bin/console cache:clear</kbd></li>
+        </ul>
+    {% elseif exception.class == "Doctrine\\DBAL\\Exception\\InvalidFieldNameException" %}
+        <i>Invalid database schema.</i> Try following things:
+        <ul>
+            <li>Run <kbd>php bin/console doctrine:migrations:migrate</kbd> to upgrade database schema</li>
+            <li>Run <kbd>php bin/console cache:clear</kbd></li>
+        </ul>
+    {% else %}
+        You could try following things, if this error is unexpected:
+        <ul>
+            <li>Check <code>var/log/prod.log</code> for additional informations</li>
+            <li>Run <kbd>php bin/console cache:clear</kbd> to clear cache</li>
+        </ul>
+    {% endif %}
+{% endblock %}