ReactCompositeComponent.mountComponent needs a try/finally

[sophiebits]

Probably it should always set _compositeLifeCycleState to null, or else React gets confused when exceptions are thrown in render (or in mountComponent, etc).

[sophiebits]

Made better by a06de4b but not entirely. In particular, the lifecycle state doesn't get changed back which gives you the confusing

Error: Invariant Violation: replaceState(...): Cannot update while unmounting component or during an existing state transition (such as within `render`).

if you make another state update. Anyone have good ideas for what the behavior should be in this case?