Produce notification when (all) metadata sources error #277
Description
I just had a situation where none of the configured metadata sources where reachable. The editor gave no indication that it saw a 502 for any request and behaviorally made the impression that I was working with an empty instance.
This is not desirable. When nothing can be read, chances are that nothing can be submitted either -- leading to potential loss of work.
Maybe it is necessary to introduce a dedicated "heartbeat" request: a request that is done (maybe every minute or so) to verify that the expected service(s) are still available with the known set of credentials.