fix: error rate panel shows 0% when no 5xx errors exist

Use $__rate_interval instead of fixed 5m, sum() to aggregate across
instances, and 'or on() vector(0)' so the numerator falls back to 0
when no 5xx series exist instead of leaving the panel blank.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>