BAU Log error message when rendering error page

If we catch an error and render the error page, we do not currently
log the error message. The `error` always evaluates as an empty
JSON object when we attempt to log it so it is not included. Instead,
log the error message.

app/utils/response-router.js

@@ -274,7 +274,7 @@ function logErrorPageShown (page, reason, loggingFields, error) {
   logger.info('Rendering error response', {
     page,
     reason,
-    error,
+    error: error && error.message,
     ...loggingFields
   })
 }