refactor: move RateLimitExceeded to roundup.cgi.exceptions RateLimitExceeded is an HTTP exception that raises code 429. Move it to roundup.cgi.exceptions where all the other exceptions that result in http status codes are located. Also make it inherit from HTTPException since it is one. Also add docstrings for all HTTP exceptions and order HTTPExceptions by status code. BREAKING CHANGE: if somebody is importing RateLimitExceeded they will need to change their import. I consider it unlikely anybody is using RateLimitExceeded. Detectors and extensions are unlikely to raise RateLimitExceeded. So I am leaving it out of the upgrading doc. Just doc in change log.

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