731 improve errorhandling

[mweststrate]

Fixes #731

• computed values that throw, store the exception and throw it to the next consumer(s). They keep tracking their data, so they are able to recover
• reactions will always catch their exceptions, and just log the error. They will keep tracking their data, so they are able to recover
• the disposer of a reaction exposes an onError(handler) function to be able to attach custom error handling logic to an reaction (this intercepts the default behavior)
• extras.onReactionError can be used to register a global onError handler for reactions (will fire after spy "error" event

[coveralls]

Coverage decreased (-0.2%) to 96.526% when pulling daf2282 on 721-improve-errorhandling into 7f859eb on mobx3.