Improve error handling and make fetch error handling behave like fetch

[hansottowirtz]

Problem

• Axios throws on HTTP errors (non-2xx), fetch does not
• There's no way to listen for PostHog errors
• shutdownAsync throws when the associated flush throws, but the flush interval ignores errors
• retriable delays even if everything succeeded

Changes

• I added validateStatus: () => true to Axios. This makes sure Axios doesn't throw any HTTP errors.
• I added two errors: PostHogFetchHttpError (non-2xx status) and PostHogFetchNetworkError (fetch network error)
• I added .on('error', (err) => void) to the API
• I made shutdownAsync ignore fetch errors. They should be handled with .on('error', ...) from now on.
• I fixed retriable by only delaying when multiple tries are needed

Release info Sub-libraries affected

Bump level

• [ ] Major
• [x] Minor
• [ ] Patch

Libraries affected

• [x] All of them
• [ ] posthog-web
• [ ] posthog-node
• [ ] posthog-react-native

Changelog notes

• Fixed fetch compatibility by aligning error handling
• Added two errors: PostHogFetchHttpError (non-2xx status) and PostHogFetchNetworkError (fetch network error)
• Added .on('error', (err) => void)
• shutdownAsync now ignores fetch errors. They should be handled with .on('error', ...) from now on.

[hansottowirtz]

Thanks! Yes, uncaught errors and timeouts handling in our analytics stack is pretty important for us. We're using Cloudflare Workers and we'll do something like .on('error', (err) => bugsnag.capture(err)).

[hansottowirtz]

Linting now also fixed :)

[hansottowirtz]

Also, I'm not sure if this should be a major or minor update, but you could consider the new errors a breaking change.