Invalid migrations being ignored by the migrate process plugin: migration_lookup

Problem/Motivation

Discovered at #3138795: d6_term_node_revision references non-existent migration

process:
  vid:
    -
      plugin: migration_lookup
      migration:
        - d6_node_copmplete
        - d6_node_revision
      source: vid
    -
      plugin: node_complete_node_revision_lookup
    -
      plugin: skip_on_empty

Looks like invalid migrations being ignored by the MigrateProcessPlugin: migration_lookup in \Drupal\migrate\Plugin\migrate\process\MigrationLookup::transform()

Proposed resolution

Validating migration IDs in migration_lookup process plugin before proceeding.

Or treat it more generic by enlarging the scope and rescope this to a meta issue to valid similar invalid config items across the migrate system.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet