Cryptic error when using .bsts with arches

When trying to load a pipeline with an element specifying the old 'arches' statements, a cryptic error is emitted:

KeyError: "Key not found in the first mapping: 'kind'"

I'd expect something more along the lines of:

Error loading pipeline: [arches.bst [line 3 column 1]]: Unexpected key: arches

Repro'd using empty project and single arches.bst:

kind: import
description: Import tar
arches:
  x86_64:
    sources:
    - kind: tar
      url: https://server.example/tar.tar.gz
      base-dir: ""

Tested at a243a333, '.gitlab-ci.yml: Issue #100 (closed) - Avoid installing setup.py test_requires'.

Broken since 4488ddf5, '_yaml.py: Implement array composition directives'.