Bad entry in overrides should be more noticeable

[bboreham]

Symptom:

level=error ts=2020-08-13T10:03:45.497348374Z caller=manager.go:85 msg="failed to load config" err="yaml: unmarshal errors:\n  line 58: key \"redacted\" already set in map"

I need sirens going off and lights flashing, because now it has dropped all of my overrides and data is being unfairly rejected.

Related: #2957, #1433

[bboreham]

In the community meeting @gouthamve pointed me at cortex_runtime_config_last_reload_successful

[bboreham]

Also it was suggested that a failure on first load should be a fatal error, which would halt a rolling update.

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[pracucci]

Still valid

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[pracucci]

Still valid

[stale]

This issue has been automatically marked as stale because it has not had any activity in the past 60 days. It will be closed in 15 days if no further activity occurs. Thank you for your contributions.

[pracucci]

Done in #3707.