Handle errors during task execution by storing in execution.el
* execution.el (execution-result::error): new slot; stores error data
  that is generated during execution
  (execution-result::has-result-p): new method; check for result
  (execution-result::has-error-p): new method; check for error data
  (execution-result::future-done-p): use `has-result-p' and
  `has-error-p'
  (execution-blocking-result::initialize-instance): store signaled
  errors into error slot of result object
  (execution-result-idle::initialize-instance): likewise
  (execution-result-timeout::initialize-instance): likewise
  (execution-result-thread::initialize-instance): likewise
* TODO (how to handle signals): marked as DONE