text: Title: Fail CI fast on example failure
  Body: Instead of running all notebook examples and then looking for errors as is currently done, we should fail fast. Fail the CI build as soon as one of the examples produces an error.
  
  Some discussion here: https://github.com/scikit-optimize/scikit-optimize/pull/208#issuecomment-242785338 also on how the current failure detection is done.
  