Rescue elasticsearch unavailable exception
When ES is unhealthy, this exception is raised when querying for logs. We should rescue and present a clear error message to the user
2020-02-10_16:39:07.03286 rails-background-jobs : 2020-02-10T16:39:07.032Z 60975 TID-ovm90l5db WARN: Elasticsearch::Transport::Transport::Errors::ServiceUnavailable: [503] {"error":{"root_cause":[{"type":"cluster_block_exception","reason":"blocked by: [SERVICE_UNAVAILABLE/1/state not recovered / initialized];"}],"type":"cluster_block_exception","reason":"blocked by: [SERVICE_UNAVAILABLE/1/state not recovered / initialized];"},"status":503}
2020-02-10_16:39:07.03287 rails-background-jobs : 2020-02-10T16:39:07.032Z 60975 TID-ovm90l5db WARN: /usr/local/lib/ruby/gems/2.6.0/gems/elasticsearch-transport-6.8.0/lib/elasticsearch/transport/transport/base.rb:205:in `__raise_transport_error'
2020-02-10_16:39:07.03287 rails-background-jobs : /usr/local/lib/ruby/gems/2.6.0/gems/elasticsearch-transport-6.8.0/lib/elasticsearch/transport/transport/base.rb:323:in `perform_request'
2020-02-10_16:39:07.03288 rails-background-jobs : /usr/local/lib/ruby/gems/2.6.0/gems/elasticsearch-transport-6.8.0/lib/elasticsearch/transport/transport/http/faraday.rb:20:in `perform_request'
2020-02-10_16:39:07.03289 rails-background-jobs : /usr/local/lib/ruby/gems/2.6.0/gems/elasticsearch-transport-6.8.0/lib/elasticsearch/transport/client.rb:143:in `perform_request'
2020-02-10_16:39:07.03290 rails-background-jobs : /usr/local/lib/ruby/gems/2.6.0/gems/elasticsearch-api-6.8.0/lib/elasticsearch/api/actions/search.rb:130:in `search'
2020-02-10_16:39:07.03291 rails-background-jobs : /Users/ak/Work/gitlab/gitlab/ee/lib/gitlab/elasticsearch/logs.rb:23:in `pod_logs'