Show error message if checkbox wasn’t checked

Because we were redirecting in all cases the error message wasn’t being
shown.

This commit changes the endpoint to respond with content (including an
error message) if the `POST` is not successful.

app/main/views/broadcast.py

@@ -22,6 +22,15 @@ from app.models.broadcast_message import BroadcastMessage, BroadcastMessages
 from app.utils import service_has_permission, user_has_permissions
 
 
+def get_back_link_endpoint():
+    return {
+        'main.view_current_broadcast': '.broadcast_dashboard',
+        'main.view_previous_broadcast': '.broadcast_dashboard_previous',
+        'main.view_rejected_broadcast': '.broadcast_dashboard_rejected',
+        'main.approve_broadcast_message': '.broadcast_dashboard',
+    }[request.endpoint]
+
+
 @main.route('/services/<uuid:service_id>/broadcast-tour/<int:step_index>')
 @user_has_permissions()
 @service_has_permission('broadcast')
@@ -389,17 +398,11 @@ def view_broadcast(service_id, broadcast_message_id):
                 broadcast_message_id=broadcast_message.id,
             ))
 
-    back_link_endpoint = {
-        'main.view_current_broadcast': '.broadcast_dashboard',
-        'main.view_previous_broadcast': '.broadcast_dashboard_previous',
-        'main.view_rejected_broadcast': '.broadcast_dashboard_rejected',
-    }[request.endpoint]
-
     return render_template(
         'views/broadcast/view-message.html',
         broadcast_message=broadcast_message,
         back_link=url_for(
-            back_link_endpoint,
+            get_back_link_endpoint(),
             service_id=current_service.id,
         ),
         form=ConfirmBroadcastForm(
@@ -442,6 +445,16 @@ def approve_broadcast_message(service_id, broadcast_message_id):
         ))
     elif form.validate_on_submit():
         broadcast_message.approve_broadcast()
+    else:
+        return render_template(
+            'views/broadcast/view-message.html',
+            broadcast_message=broadcast_message,
+            back_link=url_for(
+                get_back_link_endpoint(),
+                service_id=current_service.id,
+            ),
+            form=form,
+        )
 
     return redirect(url_for(
         '.view_current_broadcast',