[Bug]: Better error message for specifying inputs on chain pipeline in Beam YAML

[Polber]

What happened?

When specifying explicit inputs to transforms in Beam YAML, the error message is difficult to decipher.

For example, running the following pipeline:

pipeline:
  type: chain
  transforms:
    - type: Create
      config:
        elements:
          - id: 1
    - type: LogForTesting
      input: Create

will result in the following error message:

jsonschema.exceptions.ValidationError: 'Create' should not be valid under {} around line 475

Failed validating 'not' in schema['properties']['pipeline']['allOf'][2]['then']['properties']['transforms']['items']['properties']['input']:
    {'not': {}}

On instance['pipeline']['transforms'][1]['input']:
    'Create'

This should instead print something like "Cannot define explicit inputs on chain pipelines"

Issue Priority

Priority: 2 (default / most bugs should be filed as P2)

Issue Components

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