* unlock the state if Context() has an error, exactly as backend/remote does today
* terraform console and terraform import will exit before unlocking state in case of error in Context()
* responsibility for unlocking state in the local backend is pushed down the stack, out of backend.go and into each individual state operation
* add tests confirming that state is not locked after apply and plan

* backend/local: add checks that the state is unlocked after operations

This adds tests to plan, apply and refresh which validate that the state
is unlocked after all operations, regardless of exit status. I've also
added specific tests that force Context() to fail during each operation
to verify that locking behavior specifically.