Join: A better reaction to errors when joining by token in the URL

owl/templates/join-token.html

@@ -4,11 +4,12 @@
 	<h2>Join a new course{% if course %}: {{ course.name }}{% endif %}</h2>
 
 	{% if error %}
-	<p class=error>{{ error }}
-	{% endif %}
-
+		<p class=error>{{ error }}</p>
+		<p><a href='{{ url_for('index') }}' class=button>Go to main page</a></p>
+	{% else %}
 		<form method="POST" action="?">
 			{{ form.csrf_token }}
 			<input type=submit class=write value='Join'>
 		</form>
+	{% endif %}
 {% endblock %}