fix logic for unlock on unlocked lock calls

[shcheklein]

Fixes 'NoneType' object has no attribute 'close' on running multiple repro in parallel when they compete for resources and one of them fails to acquire the lock back.

• Double unlock is not allowed and now raises an explicit better DvcException. This is aligned with usual lock semantics and we want to keep as much of it as possible so that we detect missing lock() calls, or some misplaced unlocks, etc.
• Unlock on an unlocked is allowed only if this lock failed last time we tried to acquire it. This is unfortunate, but should be more or less safe. This is the best we can do considering the way we lock, unlock stuff in stages now.