Rate limit improvements

[inducer]

Summary of changes

Two changes here, happy to split if you would prefer.

• First, with my Discourse instance (v2.8.13 on nginx, in case it makes a difference), I encountered rate-limited responses that did not have a Content-Type header. The first commit makes it so that the code takes these in stride.
• The second commit includes information from the Discourse-Rate-Limit-Error-Code, which makes it easier to debug which limit is encountered. Also, logging happens before waiting, to provide feedback on what is happening.

Checklist

• Changes represent a discrete update (-ish)
• Commit messages are meaningful and descriptive
• Changeset does not include any extraneous changes unrelated to the discrete change

[bennylope]

Thank you and sorry this wasn't merged earlier!