Communicate when there is a missing plugin declaration rather than crashing

Problem/Motivation

When running `drush migrate:status` I get the error: Migrate Module: TypeError: Cannot access offset of type string on string in /var/www/docroot/core/modules/migrate/src/Plugin/Migration.php on line 744.

Steps to reproduce

Make a basic error in your YML file, in which you accidentally omit the plugin:

process:
  langcode:
    #plugin: default_value
    default_value: und