handle developer errors from callbacks separate from fetch errors

Currently we share the same catch clause for `fetch()` and for the `shouldRetry()` here: https://github.com/adobe/node-fetch-retry/blob/3e512e5a6a892e0bd51c862d8fd1abdbdf1b4c57/index.js#L217-L227

shouldRetry includes calling the callbacks `retryOnHttpError()` and `retryOnHttpResponse()`. These callbacks could have developer errors on which we should not retry blindly.

Instead, we should separate the error handling, and for errors on the callbacks pass them through as clear errors (and do not retry) so the developers can fix their code.