Error response bodies in fingerprints are discarded by Sentry

In this issue for example:

Since error messages are sometimes ignored, the actual fingerprint in use for issue grouping looks like this:

If we have, on the same endpoint URL, two different causes of the same HTTP error status code, we would only see one Sentry issue (and might miss one of the two bugs). We can try fixing this just with a JSON.stringify to ensure the body is always a string.