mgr/dashboard: Drop invalid RGW client instances, improve logging
1. Log which settings are used by the backend to connect to a RGW service. This will make it easier to check this in case of problems.
2. After removing a RGW service the UI will show an incorrect error message
because the backend is caching the instance containing the connection information. Showing a 'RGW REST API failed request ...' is incorrect, the message should be 'No RGW found'.
Dropping the invalid instance will show the correct error message when the backend tries to establish a new connection the next time.