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.