- Allow Ci::Config::Entry::Node to capture warnings and
  return warnings for all its descendants.
- Bubble creation warnings up to YamlProcessor and make
  it handle warnings by returning them as part of the
  ValidationError.
- Add YamlProcessor#error! helper to consistently report
  errors and warnings for post-syntax validations.
- Make Ci::CreatePipelineService aware of any creation
  warnings and new creation errors by appending them to
  the pipeline object being created.