devtool: fix handling of errors during task execution

devtool: fix handling of errors during task execution

* If an error is logged while executing a task, we need to ensure we
  exit instead of assuming everything went OK.
* If we receive CookerExit, the server is shutting down and we need to
  stop waiting for events and probably exit (knotty does this). This
  will occur if an exception or bb.fatal() happens during an event
  handler.

This fixes a couple of issues highlighted when using devtool upgrade or
modify on a non-supported recipe with intel-iot-refkit together with
bitbake master, but I'd be very surprised if it were hard to reproduce
in other scenarios.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>