text: Title: Disguise database connection errors as 503?
  Body: When the database connection cannot be established, this is usually because of the deployment process restarting the cluster. Currently, this leads to `500 Internal Server Error` due to an error raised by the local database client library. As the error cause is known and intended, it would be justified to replace this with `503 Service Unavailable`.
  
  The question is for which error codes to do that. Here's the server error codes reference: https://mariadb.com/kb/en/library/mariadb-error-codes/ However, errors generated by the database server (as opposed to connection errors on the client side) are real errors and should not be disguised.
  
  This is the client error code reference: https://dev.mysql.com/doc/refman/8.0/en/client-error-reference.html The following list appears reasonable: 2002, 2003, 2005, 2007, 2009, 2026
  
  @nils-wisiol 