Display notification that max concurrent sessions has been reached (GRN-41) (#317)

* <Added max concurrent sessions check>

* <Refactored some exception logic>

* <Fixed code style>

* <Generalized BBBException error message>

app/views/rooms/show.html.erb

@@ -13,6 +13,14 @@
 # with BigBlueButton; if not, see <http://www.gnu.org/licenses/>.
 %>
 
+<% unless flash.empty? %>
+  <%= render "shared/error_banner" do %>
+    <% flash.each do |key, value| %>
+      <%= content_tag :div, value, class: "flash #{key} d-inline" %>
+    <% end %>
+  <% end %>
+<% end %>
+
 <div class="background pb-1">
   <div class="container">
     <div class="row pt-9">