Keep min. 30 seconds for replay in error mode

Currently, when in error mode, we keep _up to_ 60 seconds of replay recording for when an error happens.

Basically, what happens is:

* rrweb is instructed to do a full checkout every 60 seconds
* The event buffer will generally discard anything buffered before when it receives a full checkout
* When an error occurs, we send the whole buffer
* This means we send anything from 0-60 seconds of replay data

IMHO this can be unexpected (it was unexpected to me, I thought it would always keep the last 60s 😅 ). I propose that we change this to keep a min. of 60 seconds (except at the very beginning of the replay, of course. So we'd send anywhere from 60-120 seconds of replay data.