thread state coordinator: added error callbacks, cleaned up tests.

ThreadStateCoordinator changes:
* Most commands that run in the queue now take an error handler that
  will be called with an error string if an error occurs during processing.
  Errors generally stop the operation in progress.  The errors are checked
  at time of execution.  This is intended to help flush out ptrace/waitpid/state management
  issues as quickly as possible.

* Threads now must be known to the coordinator before stops can be reported,
  resumes can be requested, thread deaths can be reported, or deferred stop
  notifications can be made.  Failure to know the thread will cause the coordinator
  to call the error callback for the event being processed.  Threads are introduced
  to the system by the NotifyThreadCreate method.

* The NotifyThreadCreate method now takes the initial state of the thread being
  introduces to the system.  We no longer just assume the thread is running.

The test cases were cleaned up, too:
* A gtest test fixture is now used, which allows creating less verbose helper
  methods that setup common pieces of callback code for some method invocations.
  Net result: the tests are simpler to read and shorter to write.