views: Check for empty list of boot modes

In set boot mode, show an error if server replies with an empty list of
boot modes

ogcp/views.py

@@ -1216,6 +1216,10 @@ def action_mode():
                   category='error')
             return redirect(url_for('commands'))
         available_modes = [(mode, mode) for mode in r.json()['modes']]
+        if not available_modes:
+            flash(_('no boot templates are available in the server'),
+                  category='error')
+            return redirect(url_for('commands'))
         form.boot.choices = list(available_modes)
 
         form.ok.render_kw = { 'formaction': url_for('action_mode') }