Standardize error return from HTTP requests in genqlient

[wwzeng1]

Purpose

This pull request aims to standardize the error handling for HTTP requests in the genqlient library. The current implementation returns a generic error message that makes it difficult to handle specific error scenarios, such as retrying on rate limit errors.

Fixes #333

I have:

• Written a clear PR title and description (above)
• Signed the Khan Academy CLA
• Added tests covering my changes, if applicable
• Included a link to the issue fixed, if applicable
• Included documentation, for new features
• Added an entry to the changelog

[jagregory]

This would be great to get merged. It's quite painful to handle HTTP errors at the moment. Do you need any help @wwzeng1?