`capture_exceptions=False` by default on new contexts if posthog is not capturing exceptions

[iloveitaly]

• If you aren't using posthog for exceptions, that preference is not respected when using the new contexts API. You have to with posthog.new_context(capture_exceptions=False): on each and every context.
• Also, there's not a way to easily set the default_client which is used by the posthog contexts
• default_client is not documented in the python documentation