There are some additional details in https://tickets.metabrainz.org/browse/LB-1107 but I'm splitting this out as a separate issue so that 1107 can focus on the 503 errors.

My last.fm account has ~55k scrobbles.  When importing it and looking the browser console, I see many requests get a 429 response, but the import appears to just ignore those errors and continue.  As a result, many (in my case, ~16k listens) were missing after the import.

Proposed fix:

• the import should ideally use the rate limit information in the response headers to delay submit-listen requests so as not to hit the rate limit
• if a request does fail with a 429 response code, it should be retried, rather than silently ignored