Adding a schedule fails silently

[a1exsh]

Spotted on 1.3.0-BETA-20180830 (8dfc08c)

Trying to add a new schedule for a keyspace which already has one fails silently in the UI and the logs only show this: POST /repair_schedule?... i.c.r.RepairScheduleResource - first schedule activation will be: 2018-09-17T09:00:00Z

I can understand the reason, but I would expect some feedback in the UI (and in the logs).

┆Issue is synchronized with this Jira Task by Unito ┆Issue Number: K8SSAND-370