Make observers handle their own errors

[raquo]

As discussed in gitter with @mattjacobus

• Add handleObserverErrors to Observer factories that let you handle errors like withRecover and fromTry
• Default that param to true (breaking change!) so that Observer's onError callback can process an error originating in its onNext callback
• When processing an observer error, internally set handleObserverErrors flag to false, so that errors in error handling logic fall through as unhandled (to prevent infinite loops)
• Not sure if this will make it into v0.12.0, swamped with other prs and issues atm
• Note: check the Airstream error handling docs first