Show failure status in recommender sidebar

[reckart]

Is your feature request related to a problem? Please describe. When a recommender fails, one can see it mainly in the recommender log that is accessible from the sidebar, but that is a bit hidden.

Describe the solution you'd like Markers should be shown for the individual recommenders in the lower part of the sidebar.

• [ ] Show a warning in the recommender sidebar if a recommender is re-using recommendations due to failure
• [ ] Show an error in the recommender sidebar if there are no recommendations due to failure and no previous suggestions can be re-used

[reckart]

I actually believe that the sidebar meanwhile indicates failures.. needs verification.