create alert and dashboard for sidekiq exceptions

We currently have very little visibility into this and it would be nice to add this to our sidekiq stats dashboard.

• azure - https://performance.gitlab.net/d/000000154/sidekiq-stats?refresh=5m&orgId=1
• gcp - https://dashboards.gitlab.net/d/000000154/sidekiq-stats?refresh=5m&orgId=1

During the last failover rehearsal we saw a large number of exceptions we should alert on. https://dashboards.gitlab.net/d/l8ifheiik/geo-status?panelId=33&fullscreen&orgId=1&from=1533223456362&to=1533250079043