Show task events in the UI

[Gerrrr]

Right now if the allocation failed before starting (e.g. docker driver could not pull the image) users have no way to find out what actually happened.

Two suggestions on how to show it:

1. Just have a place in the UI to put all the task events from the API.
2. Get only the events that indicate a failure (e.g. FailsTask = true or ExitCode = 1) and show them through the error notifications, like it is done when we start an instance.

IMO the 2nd way is better because users normally are not interested in those events as long as everything runs fine.

Looks like it is even possible to categorize the task errors since the task event has SetupError, DriverError, etc fields.