Improve error handling for settings. Instead of failing with an exception, an alert is displayed on the main page when something isn't set properly.

app/YtManagerApp/templates/YtManagerApp/index.html

@@ -24,6 +24,8 @@
         </div>
     </div>
 
+    {% include 'YtManagerApp/index_errors_banner.html' %}
+
     <div class="row">
 
         <div class="col-3">

app/YtManagerApp/templates/YtManagerApp/index_errors_banner.html

@@ -0,0 +1,24 @@
+{% if config_errors %}
+    <div class="alert alert-danger alert-card mx-auto">
+        <p>Attention! Some critical configuration errors have been found!</p>
+        <ul>
+            {% for err in config_errors %}
+                <li>{{ err }}</li>
+            {% endfor %}
+        </ul>
+        <p>Until these problems are fixed, the server may have encounter serious problems while running.
+            Please correct these errors, and then restart the server.</p>
+    </div>
+{% endif %}
+
+{% if config_warnings %}
+    <div class="alert alert-warning alert-card mx-auto">
+        <p>Warning: some configuration problems have been found!</p>
+        <ul>
+            {% for err in config_warnings %}
+                <li>{{ err }}</li>
+            {% endfor %}
+        </ul>
+        <p>We recommend that you fix these issues before continuing.</p>
+    </div>
+{% endif %}

app/YtManagerApp/templates/YtManagerApp/index_unauthenticated.html

@@ -3,6 +3,8 @@
 
 {% block body %}
 
+    {% include 'YtManagerApp/index_errors_banner.html' %}
+
     <h1>Hello</h1>
     <h2>Please log in to continue</h2>