How do rate limits affect migration?

[aaronsteers]

There is some text around rate limits and throttling but I could not find specific answer on:

1. What's the behavior when one or more rate limits are reached? Does the process fail and is it directly resumable if so? (Conversely, would some rate limits result in fatal error and a need to "start over"?)
2. Does anyone have experience with intentionally throttling to avoid those limits being reached?

Related: multi-project migration question: #137 .