Error from setting favorite not caught

[ascheman]

We currently see Server errors (500) on latest and testing due to a DB error when setting a new talk preference (favorite). The server returns:

{"status":500,"reason":{"timestamp":"2019-03-03T17:55:12.780+0000","status":500,"error":"Internal Server Error","message":"could not execute statement; SQL [n/a]; constraint [id]; nested exception is org.hibernate.exception.ConstraintViolationException: could not execute statement","path":"/rest/preferences"}}

But this is not caught in the PWA frontend.

I will try to fix the problem on the backend ASAP. Nevertheless we should see a better error handling in the frontend as well.

[steffchep]

@srose @ascheman a toast message should appear on error now, can you check if this works and is sufficient?