APM server 8.0+ returns 500 or 599 instead of 503

## Describe the bug
APM server 8.0+ returns 500 (timeout on connection - timeout is specified by the client) or 599 (not sure when this happens) instead of 503 (queue full). Doesn't look like it matters much to the agent, but we do have a custom error string for 503s in ApmServerConfigurationSource, so we may want to remove or add the other options