Change S3 access error level to debug

[txau]

Every now and then there are small intermittences in the S3 service, resulting in requests responding with a 500 and flooding our logs with critical notifications.

If we can assume eventual failure of S3, we don't need to record these logs as critical.

1. Change the level of the log from error to debug
2. Change the response code from 500 to 503

The 503 Service Unavailable response code means that the server is currently not ready to handle the request. This is a common occurrence when the server is down for maintenance or is overloaded.

When looking at things SEO-wise the 503 Service Unavailable status code means that the server is unavailable and the visitor, bot or human, is asked to return again at a later time. This could be because of either server maintenance or server overload and search engines know to come back and check the availability later.