Fix dropped logs due to incorrect error field type

Kibana validates log lines by index patterns and "error" is supposed to be an object. Since error is a string in logstaher's world we are renaming it to "message" which will be accepted as string.

Co-authored-by: Neamah Al-Selwi <neamah.alselwi@digital.cabinet-office.gov.uk>
Co-authored-by: Tuomas Nylund <tuomas.nylund@digital.cabinet-office.gov.uk>
Co-authored-by: Laura Ghiorghisor <laura.ghiorghisor@digital.cabinet-office.gov.uk>

lib/govuk_app_config/govuk_json_logging.rb

@@ -50,6 +50,11 @@ def self.configure
     # Elasticsearch index expect source to be an object and logstash defaults
     # source to be the host IP address causing logs to be dropped.
     Rails.application.config.logstasher.source = {}
+    # Elasticsearch index expect error to be an object and logstash defaults
+    # error to be a string causing logs to be dropped.
+    Rails.application.config.logstasher.field_renaming = {
+      error: :message,
+    }
 
     Rails.application.config.logstasher.logger = Logger.new(
       $stdout,