Add error message to failed requests

[iwex]

Sometimes we don't log useful info about an error. In my case, it was hard to debug that {"detail":"envelope exceeded size limits for type 'event' (https://develop.sentry.dev/sdk/envelopes/#size-limits)"}

This PR will add an error message for some unsuccessful requests.

I can't find an API for possible response - https://develop.sentry.dev/sdk/envelopes/ So I guess we can just use general info about error.