We should handle this error if possible, and display an appropriate message to the user asking them to retry their upload later:

To quote IA about this status code:

S3 returns the 503 slowdown messages when our content ingest layer queue starts getting very large
and a backlog of ingests have accumulated.

This is usually caused by some sort of temporary trouble on the ingest side of our storage system (in this
case, a fan outage caused, high temperatures, and automatic machine off lining took place, significantly
reducing the ingest rate).

These conditions are very rarely the end user's (or api clien's, in your case) fault.