fix(roster): wrong alert class name

roster/templates/roster/currshow.html

@@ -17,7 +17,7 @@
   For a description of what these units are,
   see the <a href="{% url "catalog" %}">unit catalog</a>.
   {% if form.errors %}
-    <div class="alert alert-error">
+    <div class="alert alert-danger">
       <ul>
         {% for field in form %}
           {% if field.errors %}<li>{{ field.label }}: {{ field.errors|striptags }}</li>{% endif %}