tests/testutils/runcli.py: Augment the Result object to store task errors

Before this, we only stored the last exception to have been raised in the main
process, now we additionally provide some Result members allowing tests to
inspect a machine readable error `domain` and `reason` describing why
a task has "failed".

This adds some new APIs to the Result() object for tests:

  assert_main_error() - asserts the nature of the main buildstream error
  assert_task_error() - asserts the nature of the error from a child task