Make sure people can't join an event with limited participants

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/mobilizon_web/resolvers/event.ex

@@ -170,6 +170,9 @@ defmodule MobilizonWeb.Resolvers.Event do
            |> Map.put(:actor, Person.proxify_pictures(actor)) do
       {:ok, participant}
     else
+      {:maximum_attendee_capacity, _} ->
+        {:error, "The event has already reached it's maximum capacity"}
+
       {:has_event, _} ->
         {:error, "Event with this ID #{inspect(event_id)} doesn't exist"}