refactor(tracing): remove EventFilter::exception and always attach exception

[lcian]

Before this change, when capturing something with e.g. tracing::warn!(error = std_dyn_err(&e), "Something went wrong"); the following would happen based on the EventFilter:

• EventFilter::Exception -> exception_from_event (includes error in the exception field of the Event)
• EventFilter::Event -> event_from_event (ignores error)
• EventFilter::Breadcrumb -> breadcrumb_from_event (ignores error)
  So, both Event and Breadcrumb would counterintuitively ignore the field.

With this change:

• we remove EventFilter::Exception and exception_from_event in favor of EventFilter::Event and event_from_event, which will behave as the old exception_from_event (attaching error, if any)
• we attach the error field as structured data in breacrumbs. It's attached as a list of errors from the outermost to the innermost, mimicking the way it would be reported in a Sentry issue.

Closes #702

[Swatinem]

just a heads up that this will be a breaking change.