Allow exceptions to escape CallController#execute

The goal is to allow spec runs to invoke controllers and get an exception when it occurs.  The current behavior catches the exception and sends it through the exception handler, which often will silently swallow it, especially if the tests complete and the process exits before the exception is worked from the queue.

This commit still has two broken tests, so more discussion is necessary.
Switch tests from private #execute! to #exec
Change tests to reflect expectation that #exec propagates exceptions upward
Spec to ensure exception behavior on #execute_controller
Remove accidental focus
Ensure the call is still alive after an exception
Use latch to synchronize the test
Merge pull request #490 from adhearsion/feature/better_exception_handling_for_rspec

Allow exceptions to escape CallController#execute