Fixed the first half of #750

The fixes the first half of #750, namely the test for exception propagation from a Task's `call()` method:

 * Removed the `FailedTask` handler in the test's Driver. This means that we now forward the errors correctly to the client.
 * Fixed a log message in `FailTaskStart`
 * Updated `EvaluatorManager` logging
Merge pull request #823 from Microsoft-CISL/weimer_750a

Fixed the first half of #750