Improve UX for error messaging for dynamic inputs

The following discussion from !214046 (merged) should be addressed:

• @avielle started a discussion: (+2 comments)

  In the example config from the MR description, this input:

      # Level 3: Depends on environment + deployment_type
      canary_percentage:
        description: 'Canary rollout percentage'
        rules:
          - if: $[[ inputs.deployment_type ]] == 'canary' && $[[ inputs.environment ]] == 'production'
            options: ['10', '25', '50', '75']
            default: '25'
          - if: $[[ inputs.deployment_type ]] == 'canary' && $[[ inputs.environment ]] == 'staging'
            options: ['50', '100']
            default: '50'
          - if: $[[ inputs.deployment_type ]] == 'canary'
            options: ['100']
            default: '100'
          - options: []

  is giving me this error in the CI editor

  This GitLab CI configuration is invalid: `canary_percentage` input: `` cannot be used because it is not in the list of allowed options

  Is that what we expect?