Don't blow up on exceptions in `traces_sampler` (and other hooks)

[sentrivana]

If an exception happens in a user's traces_sampler (or other callback), we shouldn't crash.

[linear-code]

PY-2617

[alexander-alderman-webb]

We should make sure before_send_span() also doesn't crash application.
And generally any user-provided hooks that are called when using the sentry_sdk.traces.