thread state coordinator: add tests and impl to error on creation/death issues.

Added tests and impl to make sure the following errors are reported:
* Notifying a created thread that we are already tracking.
* Notifying a thread death for a thread we don't know about.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@218900 91177308-0d34-0410-b5e6-96231b3b80d8